Add pull request template

.github/pull_request_template.md

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+<!-- Thank you for submitting a pull request! Find more information in our
+development guide at
+https://github.com/trinodb/trino/blob/master/.github/DEVELOPMENT.md
+and contact us on #dev in slack. -->
+## Description
+<!-- Elaborate beyond the title of the PR as necessary to help the reviewers
+and maintainers.-->
+
+
+## General information
+
+Is this change a fix, improvement, new feature, refactoring, or other?
+
+
+Is this a change to the core query engine, a connector, client library, or the
+SPI interfaces (be specific)?
+
+
+How would you describe this change to a non-technical end user or system
+administrator?
+
+## Related issues, pull requests, and links
+<!-- List any issue that is fixed and provide links to other related PRs,
+upstream release notes, and other useful resources:
+* Fixes #issuenumber
+* Related documentation in #issuenumber
+* [Some release notes](http://usefulinfo.example.com)
+-->
+
+
+<!--
+The following sections are filled in by the maintainer with input from the
+contributor:
+
+Use :white_check_mark: or (x) or whatever really to signal selection.
+-->
+## Documentation
+
+( ) No documentation is needed.
+( ) Sufficient documentation is included in this PR.
+( ) Documentation PR is available with #prnumber.
+( ) Documentation issue #issuenumber is filed, and can be handled later.
+
+## Release notes
+
+( ) No release notes entries required.
+( ) Release notes entries required with the following suggested text:
+
+```
+# Section
+* Fix some things. ({issue}`5678`)
+```