[AIRFLOW-XXX] Add note about backwards incompatible changes (apache#4843

)

.github/PULL_REQUEST_TEMPLATE.md

@@ -30,6 +30,7 @@ Make sure you have checked _all_ steps below.
 - [ ] In case of new functionality, my PR adds documentation that describes how to use it.
   - When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
   - All the public functions and the classes in the PR contain docstrings that explain what it does
+  - If you implement backwards incompatible changes, please leave a note in the [Updating.md](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release
 
 ### Code Quality