Describe the release cycle of Stan

[WardBrian]

This document is intended to

1. document the release process as I and others have carried it out over the last several years
2. make more explicit some expectations in terms of judgement calls, things that can delay a release, etc.

It's currently in a state where I'm not sure it is ready to merge, but I could definitely use a second set of eyes to flesh things out.

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