Update release notes

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

RELEASE_NOTES.md

@@ -12,6 +12,8 @@
 
 - Added support for disabling the overflow-warning log messagess in broadcast receivers, through the `warn_on_overflow` parameter on `Broadcast.new_receiver()` calls.
 
+- Added an abstract `Receiver.close()` method for closing just a receiver.  Also implemented it for all the `Receiver` implementations in this library.
+
 ## Bug Fixes
 
 <!-- Here goes notable bug fixes that are worth a special mention or explanation -->