Prepare release notes for v1.4.0 (#346)

Author: llucax

RELEASE_NOTES.md

@@ -1,20 +1,8 @@
 # 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
 
 ### Experimental
 
 - A new predicate, `OnlyIfPrevious`, to `filter()` messages based on the previous message.
 - A new special case of `OnlyIfPrevious`, `ChangedOnly`, to skip messages if they are equal to the previous message.
-
-## Bug Fixes
-
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->