Document how we choose which browser versions to include

[ddbeck]

We should document how we choose what versions of browsers to include. This was spurred by discussion on #4467:

Yeah, this conflates a few things, but we don't really spell any of it out anywhere. There's a few facets to this, but I think the section, as written, only addresses the third (and badly):

1. We have data for major releases and we choose not to include data for prereleases (such as Safari TPs, Microsoft Edge preview builds, etc.).
2. We make an exception for browsers' prereleases where there's a well-established release process such that the unstable versions are expected to eventually become stable (i.e., Firefox and Chrome's betas and nightlies).
3. Some Node.js releases look like prereleases (i.e., 0.10 and 0.12) but aren't really because of Node.js-specific history and that's why we allow them.
4. Some non-major version number releases of Node.js include a new version of V8 that includes an ECMAScript change and we allow those versions too.

All of these things have to do with qualitative distinctions about the data we introduce into browsers/*.json and I don't think the linter can help us with any of these.

Originally posted by @ddbeck in #4467