note irctest_stable in release runbook

DEVELOPING.md

@@ -40,6 +40,7 @@ Develop branches are either used to work out implementation details in preperati
 1. Publish the release on GitHub (Releases -> "Draft a new release"); use the new tag, post the changelog entries, upload the binaries
 1. Update the `irctest_stable` branch with the new changes (this may be a force push).
 1. If it's a production release (as opposed to a release candidate), update the `stable` branch with the new changes. (This may be a force push in the event that stable contained a backport. This is fine because all stable releases and release candidates are tagged.)
+1. Similarly, for a production release, update the `irctest_stable` branch (this is the branch used by upstream irctest to integration-test against Oragono).
 1. Make the appropriate announcements:
     * For a release candidate:
         1. the channel topic