Policy addition: New files should have proper license headers.

Originally committed as revision 8825 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1629,6 +1629,9 @@ please use av_log() instead.
 @item
     Do not change code to hide warnings without ensuring that the underlying
     logic is correct and thus the warning was inappropriate.
+@item
+    If you add a new file, give it a proper license header. Do not copy and
+    paste it from a random place, use an existing file as template.
 @end enumerate
 
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