Add release note entry for DRM-protected downloads

PiperOrigin-RevId: 331539036

RELEASENOTES.md

@@ -342,6 +342,8 @@
     *   Remove support for media tunneling, random ABR and playback of
         spherical video. Developers wishing to experiment with these features
         can enable them by modifying the demo app source code.
+    *   Add support for downloading DRM-protected content using offline
+        Widevine licenses.
 
 ### 2.11.8 (2020-08-25) ###