Mention that libavcodec/ac3dec.c is GPLed.

Originally committed as revision 15226 to svn://svn.ffmpeg.org/ffmpeg/trunk

README

@@ -20,6 +20,10 @@ FFmpeg README
 * The file libavcodec/i386/idct_mmx.c is distributed under the GNU General
   Public License. It is strictly an optimization and its use is optional.
 
+* The file libavcodec/ac3dec.c is distributed under the GNU General Public
+  License.  In order for (E-)AC-3 decoding to work you need to enable GPL
+ components.
+
 * The file libavdevice/x11grab.c is distributed under the GNU General
   Public License. X11 grabbing is optional.