Add a short note about libvpx.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23394 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

LICENSE

@@ -46,3 +46,7 @@ version needs to be upgraded by passing --enable-version3 to configure.
 The nonfree external library libfaac can be hooked up in FFmpeg. You need to
 pass --enable-nonfree to configure to enable it. Employ this option with care
 as FFmpeg then becomes nonfree and unredistributable.
+
+The libvpx external library has a license that is (L)GPL incompatible. If you
+combine it with FFmpeg the resulting binaries are unredistributable. Thus you
+need to pass --enable-nonfree to configure to enable it.