Prepare release notes for v0.2.1 (#32)

Author: llucax

RELEASE_NOTES.md

@@ -2,16 +2,5 @@
 
 ## 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
-
-<!-- Here goes the main new features and examples or instructions on how to use them -->
-
-## Bug Fixes
-
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+* The generated docs now show the type of symbol in the table of contents and have cross-linking to the protobuf and channels documentation.
+* The minimum supported version of `protobuf` was downgraded to 4.21.6, so downstream projects having this older version as minimum requirements can still use this release.