Prepare release notes for v1.9.0 (#420)

Author: shsms

RELEASE_NOTES.md

@@ -1,13 +1,5 @@
 # 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.
@@ -18,7 +10,3 @@
   opt_recv: Receiver[T] | None
   recv: Receiver[T] = NopReceiver[T]() if opt_recv is None else opt_recv
   ```
-
-## Bug Fixes
-
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->