Clarify license requirements for contributions.

Originally committed as revision 9626 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1550,6 +1550,11 @@ should also be avoided if they don't make the code easier to understand.
 @section Development Policy
 
 @enumerate
+@item
+   Contributions should be licensed under the LGPL 2.1, including an
+   "or any later version" clause, or the MIT license.  GPL 2 including
+   an "or any later version" clause is also acceptable, but LGPL is
+   preferred.
 @item
    You must not commit code which breaks FFmpeg! (Meaning unfinished but
    enabled code which breaks compilation or compiles but does not work or