Update release notes

Signed-off-by: camille-bouvy-frequenz <[email protected]>

Author: camille-bouvy-frequenz

RELEASE_NOTES.md

@@ -10,9 +10,14 @@
     * Removed `list_public_trades`
     * Replaced `public_trades_stream` with `receive_public_trades`
     * `receive_public_trades` now supports an optional time range (`start_time`, `end_time`)
+* Update the `frequenz-api-electricity-trading` from >= 0.5.0, < 0.6.0 to >= 0.6.1, < 0.7.0
 
 ## New Features
 
+* Add the Public Order Book extension to the client
+    * Add the `PublicOrder` and `PublicOrderFilter` types
+    * Add the `receive_public_order()` endpoint
+
 
 ## Bug Fixes