Track release notes in the repository

[ddbeck]

Right now, release notes are in GitHub's releases page only. This is probably OK, but it would be good to put them into git. This would make writing release notes collaborative and incremental, add a layer of review, and duplicate them outside a proprietary tool.

To do this we'd need:

• copy existing release notes into a RELEASE_NOTES.md or similar
• docs for how to write and update the notes
• change the release notes script to update the file and use it as a source for adding a new release to GitHub

[queengooborg]

I believe it's safe to close this issue, as you've now done every step in that checklist?

[ddbeck]

Yes, thank you!