Add a note about breaking changes (chartjs#4555)

docs/developers/contributing.md

@@ -8,6 +8,7 @@ New contributions to the library are welcome, but we ask that you please follow
 - Check that your code will pass tests, `gulp test` will run tests for you.
 - Keep pull requests concise, and document new functionality in the relevant `.md` file.
 - Consider whether your changes are useful for all users, or if creating a Chart.js [plugin](plugins.md) would be more appropriate.
+- Avoid breaking changes unless there is an upcoming major release, which are infrequent. We encourage people to write plugins for most new advanced features, so care a lot about backwards compatibility.
 
 # Joining the project