[ANNOUNCE] New location for Gerrit release notes

[David Pursehouse]

Release notes are now kept on the Gerrit project homepage:

https://www.gerritcodereview.com/releases/README.md 

Rationale for this move is described in [1] but in short: it's now easier to update and publish release notes, and we are publishing in-progress release notes for upcoming releases.

Release notes for 2.12 onwards have been migrated to the new location. Previous releases will be done later, time permitting.

The format/layout is still up for discussion; contributions are welcome if anyone has any ideas for improvements.

[1] https://gerrit-review.googlesource.com/#/c/87070/