Add maintainance instructions

Author: yguedidi

CONTRIBUTING.md

@@ -5,6 +5,12 @@ Contributions are **welcome** and will be fully **credited**.
 
 We accept contributions via Pull Requests on [Github](https://github.com/facebook/php-graph-sdk/pull/new).
 
+The current stable major version is v5. The v6 is under active development.
+
+This means any new feature MUST target v6 (`master` branch).
+
+The v5 (`5.x` branch) is maintained only for bug fixes, node/edge updates or documentation improvements.
+
 ## Code of Conduct
 The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md)