Reset release notes after v0.5.0 release

Signed-off-by: cyiallou - Costas <[email protected]>

Author: cyiallou

RELEASE_NOTES.md

@@ -2,7 +2,7 @@
 
 ## Summary
 
-- Code relevant to fetching data and extracting state durations and alert records has been moved to another repo (`frequenz-lib-notebooks`) and has therefore been deleted from this repo.
+<!-- Here goes a general summary of what this release is about -->
 
 ## Upgrading