Update release notes

Author: cwasicki

RELEASE_NOTES.md

@@ -14,6 +14,8 @@
 - In `OrderedRingBuffer`:
   - Rename `datetime_to_index` to `to_internal_index` to avoid confusion between the internal index and the external index.
   - Remove `__setitem__` method to enforce usage of dedicated `update` method only.
+- Integer indices are supported in `OrderedRingBuffer` and `MovingWindow`.
+
 
 ## Bug Fixes