Add doc on supported platforms (influxdata#9100)

docs/SUPPORTED_PLATFORMS.md

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+# Supported Platforms
+
+Telegraf is a cross-platform application. This doc helps define which
+operating systems, distributions, and releases Telegraf supports.
+
+Telegraf is supported on Linux, FreeBSD, Windows, and macOS. It is
+written in Go which supports these operating systems and
+more. Telegraf may work on Go's other operating systems and users are
+welcome to build their own binaries for them. Bug reports should be
+submitted only for supported platforms.
+
+Golang.org has a [table][go-table] of valid OS and architecture
+combinations and the golang wiki has more specific [minimum
+requirements][go-reqs] for Go itself.
+
+[go-table]: https://golang.org/doc/install/source#environment
+[go-reqs]: https://github.com/golang/go/wiki/MinimumRequirements#operating-systems
+
+## Linux
+
+Telegraf intent: *Support latest versions of major linux
+distributions*
+
+Telegraf supports RHEL, Fedora, Debian, and Ubuntu. InfluxData
+provides package repositories for these distributions. Instructions
+for using the package repositories can be found on
+[docs.influxdata.com][repo-docs]. Bug reports should be submitted only
+for supported distributions and releases.
+
+Telegraf's Debian or Ubuntu packages are likely to work on other
+Debian-based distributions although these are not
+supported. Similarly, Telegraf's Fedora and RHEL packages are likely
+to work on other Redhat-based distributions although again these are
+not supported.
+
+Telegraf releases include .tar.gz packages for use with other
+distributions, for building container images, or for installation
+without a package manager. As part of telegraf's release process we
+publish [official images][docker-hub] to Docker Hub.
+
+Distrowatch lists [major distributions][dw-major] and tracks
+[popularity][dw-pop] of distributions. Wikipedia lists [linux
+distributions][wp-distro] by the major distribution they're based on.
+
+[repo-docs]: https://docs.influxdata.com/telegraf/latest/introduction/installation/
+[docker-hub]: https://hub.docker.com/_/telegraf
+[dw-major]: https://distrowatch.com/dwres.php?resource=major
+[dw-pop]: https://distrowatch.com/dwres.php?resource=popularity
+[wp-distro]: https://en.wikipedia.org/wiki/List_of_Linux_distributions
+
+### RHEL
+
+Red Hat makes a major release every four to five years and supports
+each release in production for ten years. Extended support is
+available for three or more years.
+
+Telegraf intent: *Support releases in RHEL production, but not in
+extended support.*
+
+Redhat publishes [release history][rh-history] and wikipedia has a
+[summary timeline][wp-rhel].
+
+As of April 2021, 7 and 8 are production releases.
+
+[rh-history]: https://access.redhat.com/articles/3078
+[wp-rhel]: https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history_and_timeline
+
+### Ubuntu
+
+Ubuntu makes two releases a year. Every two years one of the releases
+is an LTS (long-term support) release. Interim (non-LTS) releases are
+in standard support for nine months. LTS releases are in maintenance
+for five years, then in extended security maintenance for up to three
+more years.
+
+Telegraf intent: *Support interim releases and LTS releases in Ubuntu
+maintenance, but not in extended security maintenance.*
+
+Ubuntu publishes [release history][ub-history] and wikipedia has a
+[table][wp-ub] of all releases and support status.
+
+As of April 2021, Ubuntu 20.10 is in standard support. Ubuntu 18.04
+LTS and 20.04 LTS are in maintenance.
+
+[ub-history]: https://ubuntu.com/about/release-cycle
+[wp-ub]: https://en.wikipedia.org/wiki/Ubuntu_version_history#Table_of_versions
+
+### Debian
+
+Debian generally makes major releases every two years and provides
+security support for each release for three years. After security
+support expires the release enters long term support (LTS) until at
+least five years after release.
+
+Telegraf intent: *Support releases under Debian security support*
+
+Debian publishes [releases and support status][deb-history] and
+wikipedia has a [summary table][wp-deb].
+
+As of April 2021, Debian 10 is in security support.
+
+[deb-history]: https://www.debian.org/releases/
+[wp-deb]: https://en.wikipedia.org/wiki/Debian_version_history#Release_table
+
+### Fedora
+
+Fedora makes two releases a year and supports each release for a year.
+
+Telegraf intent: *Support releases supported by Fedora*
+
+Fedora publishes [release history][fed-history] and wikipedia has a
+[summary table][wp-fed].
+
+[fed-history]: https://fedoraproject.org/wiki/Releases
+[wp-fed]: https://en.wikipedia.org/wiki/Fedora_version_history#Version_history
+
+## FreeBSD
+
+FreeBSD makes major releases about every two years. Releases reach end
+of life after five years.
+
+Telegraf intent: *Support releases under FreeBSD security support*
+
+FreeBSD publishes [release history][freebsd-history] and wikipedia has
+a [summary table][wp-freebsd].
+
+As of April 2021, releases 11 and 12 are under security support.
+
+[freebsd-history]: https://www.freebsd.org/security/#sup
+[wp-freebsd]: https://en.wikipedia.org/wiki/FreeBSD#Version_history
+
+## Windows
+
+Telegraf intent: *Support current versions of Windows and Windows
+Server*
+
+Microsoft has two release channels, the semi-annual channel (SAC) and
+the long-term servicing channel (LTSC). The semi-annual channel is for
+mainstream feature releases.
+
+Microsoft publishes [lifecycle policy by release][ms-lifecycle] and a
+[product lifecycle faq][ms-lifecycle-faq].
+
+[ms-lifecycle]: https://docs.microsoft.com/en-us/lifecycle/products/?terms=windows
+[ms-lifecycle-faq]: https://docs.microsoft.com/en-us/lifecycle/faq/windows
+
+### Windows 10
+
+Windows 10 makes SAC releases twice a year and supports those releases
+for [18 or 30 months][w10-timeline]. They also make LTSC releases
+which are supported for 10 years but are intended only for medical or
+industrial devices that require a static feature set.
+
+Telegraf intent: *Support semi-annual channel releases supported by
+Microsoft*
+
+Microsoft publishes Windows 10 [release information][w10-history], and
+[servicing channels][w10-channels]. Wikipedia has a [summary
+table][wp-w10] of support status.
+
+As of April 2021, versions 19H2, 20H1, and 20H2 are supported.
+
+[w10-timeline]: https://docs.microsoft.com/en-us/lifecycle/faq/windows#what-is-the-servicing-timeline-for-a-version-feature-update-of-windows-10
+[w10-history]: https://docs.microsoft.com/en-us/windows/release-health/release-information
+[w10-channels]: https://docs.microsoft.com/en-us/windows/deployment/update/get-started-updates-channels-tools
+[wp-w10]: https://en.wikipedia.org/wiki/Windows_10_version_history#Channels
+
+### Windows Server
+
+Windows Server makes SAC releases for that are supported for 18 months
+and LTSC releases that are supported for five years under mainstream
+support and five more years under extended support.
+
+Telegraf intent: *Support current semi-annual channel releases
+supported by Microsoft and long-term releases under mainstream
+support*
+
+Microsoft publishes Windows Server [release information][ws-history]
+and [servicing channels][ws-channels].
+
+As of April 2021, Server 2016 (version 1607) and Server 2019 (version
+1809) are LTSC releases under mainstream support and versions 1909,
+2004, and 20H2 are supported SAC releases.
+
+[ws-history]: https://docs.microsoft.com/en-us/windows-server/get-started/windows-server-release-info
+[ws-channels]: https://docs.microsoft.com/en-us/windows-server/get-started-19/servicing-channels-19
+
+## macOS
+
+MacOS makes one major release a year and provides support for each
+release for three years.
+
+Telegraf intent: *Support releases supported by Apple*
+
+Release history is available from [wikipedia][wp-macos].
+
+As of April 2021, 10.14, 10.15, and 11 are supported.
+
+[wp-macos]: https://en.wikipedia.org/wiki/MacOS#Release_history