Clear release notes after release (frequenz-floss#193)

Author: florian-wagner-frequenz

RELEASE_NOTES.md

@@ -6,14 +6,11 @@
 
 ## Upgrading
 
-* The `key` parameter of the client has been renamed to `auth_key`
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
 
 ## New Features
 
-* Add HMAC generation capabilities.
-    * The new CLI option "sign_secret" can be used to provide the server's HMAC secret.
-    * The client itself now has a "sign_secret" argument in the constructor.
-    * Update documentation describing how to use the above options.
+<!-- Here goes the main new features and examples or instructions on how to use them -->
 
 ## Bug Fixes