Talk about stable branches in CONTRIBUTING.md

Summary: It was pointed out on a github issue that the process for
merges into a stable branch isn't super clear, and that this was a
pretty good place to document it. This hasn't really been an issue until
now, since we now have long-running LTS branches.

Ref. facebook#4521

{sync, type="child", parent="internal", parentrevid="1770946", parentrevfbid="1603678063185212", parentdiffid="6105947"}

Reviewed By: @JoelMarcey

Differential Revision: D1770946

Signature: t1:1770946:1420680059:78f89a907398d352dfd4006b006e2f564b16f2be

CONTRIBUTING.md

@@ -12,6 +12,12 @@ Before changes can be accepted a [Contributor Licensing Agreement](http://code.f
 
 Please add appropriate test cases as you make changes, and make sure that they pass locally before submitting your pull request; see [here](hphp/test/README.md) for more information.  All the tests are run via Phabricator, however testing locally greatly speeds up the process of accepting your changes.
 
+### Stable Version Updates
+
+We maintain up to three [stable branches](https://github.com/facebook/hhvm/wiki/Release%20Schedule) at once (the current release plus two [LTS branches](https://github.com/facebook/hhvm/wiki/Long-term-support-%28LTS%29)). To get a fix into one of those branches, first get accepted into master, as described above. Fixes are merged into master and then merged backwards into stable releases as appropriate.
+
+Then, submit another PR against the relevant stable branch(es) cherry-picking your change into that branch, with any changes needed to properly backport. Make sure to explain in the PR summary why the change should be considered for inclusion in the stable branch -- basically, make the case for why the issue the change is fixing is worse than the possible risk of what the change might break (and thus what *we* will be responsible for debugging, fixing, and maintaining).
+
 ## Quick Links
 
  * IRC: [#hhvm](http://webchat.freenode.net/?channels=hhvm) and [#hhvm-dev](http://webchat.freenode.net/?channels=hhvm-dev) on Freenode.