Add CHANGELOG entry for plan b removal

Author: lookback-hugotunius

webrtc/CHANGELOG.md

@@ -14,6 +14,7 @@ directions that should not send. [#316](https://github.com/webrtc-rs/webrtc/pull
 
 * Allow one single direction for extmap matching. [#321](https://github.com/webrtc-rs/webrtc/pull/321). API
 change for MediaEngine::register_header_extension
+* Removes support for Plan-B. All major implementations of WebRTC now support unified and continuing support for plan-b is an undue maintenance burden when unified can be used. See [“Unified Plan” Transition Guide (JavaScript)](https://docs.google.com/document/d/1-ZfikoUtoJa9k-GZG1daN0BU3IjIanQ_JSscHxQesvU/) for an overview of the changes required to migrate. [#320](https://github.com/webrtc-rs/webrtc/pull/320) by [@algesten](https://github.com/algesten).
 
 ## 0.5.1