Document mencoder deprecation in whats_new.rst

Author: efiring

doc/users/whats_new.rst

@@ -222,6 +222,13 @@ Change in the default animation codec
 The default animation codec has been changed from ``mpeg4`` to ``h264``,
 which is more efficient. It can be set via the ``animation.codec`` rcParam.
 
+Deprecated support for mencoder in animation
+--------------------------------------------
+
+The use of mencoder for writing video files with mpl is problematic;
+switching to ffmpeg is strongly advised.  All support for mencoder
+will be removed in version 2.2.
+
 
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