Add a note about license headers to the patch submission checklist.

Originally committed as revision 8793 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1671,6 +1671,9 @@ do not attach several unrelated patches to the same mail.
     patch easily?
 @item
     If you did any benchmarks, did you provide them in the mail?
+@item
+    If you added a new file, did you insert a license header? It should be
+    taken from FFmpeg, not randomly copy and pasted from somewhere else.
 @end enumerate
 
 @section Patch review process