Update contribution guidelines for RxJava

Added a warning about contributing to the 4.x branch and the future of 3.x versions.

Author: akarnokd

CONTRIBUTING.md

@@ -1,5 +1,11 @@
 # Contributing to RxJava 3.x
 
+:warning: We ask you to contribute your change to the 4.x branch because since March 2026, we are developing RxJava 4.0.0 only. It will be then decided if a backport is necessary or feasible. We do not plan to release any more 3.x versions unless there is a very good reason or a flaw that needs patching and cannot be worked around otherwise.
+
+Thank you for your consideration.
+
+------------
+
 If you would like to contribute code you can do so through GitHub by forking the repository and sending a pull request targeting the branch `3.x`.
 
 When submitting code, please make every effort to follow existing conventions and style in order to keep the code as readable as possible.