[CI-SKIP] Add note on 1.12.2 status to CONTRIBUTING.md

CONTRIBUTING.md

@@ -2,6 +2,14 @@ Contributing to Paper
 ==========================
 PaperMC has a very lenient policy towards PRs, but would prefer that you try and adhere to the following guidelines.
 
+## A notice on 1.12.2
+While we are continuing to support 1.12.2 through these initial 1.13 versions, we now
+consider it a "stable" version. As such, we will not accept Pull Requests adding major
+new features or any new API to this version. 1.12.2 is now in a "fixes only" state.
+
+Please target major feature additions and API additions to the master branch where we
+are working to make 1.13 the best it can be.
+
 ## Understanding Patches
 Patches to Paper are very simple, but center around the directories 'Paper-API' and 'Paper-Server'