Clear release notes (frequenz-floss#421)

Author: shsms

RELEASE_NOTES.md

@@ -1,12 +1,17 @@
 # Frequenz channels Release Notes
 
+## Summary
+
+<!-- Here goes a general summary of what this release is about -->
+
+## Upgrading
+
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+
 ## New Features
 
-- An experimental `NopReceiver` implementation has been added, which can be used as a place-holder receiver that never receives a message.
+<!-- Here goes the main new features and examples or instructions on how to use them -->
 
-- The experimental `OptionalReceiver` has been deprecated.  It will be removed with the next major release.  It can be replaced with a `NopReceiver` as follows:
+## Bug Fixes
 
-  ```python
-  opt_recv: Receiver[T] | None
-  recv: Receiver[T] = NopReceiver[T]() if opt_recv is None else opt_recv
-  ```
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->