Add a note about tabs + trailing whitespace to the patch submission checklist.

Originally committed as revision 8889 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1691,6 +1691,8 @@ do not attach several unrelated patches to the same mail.
     applied with @code{patch -p0}?
 @item
     Does the patch not mix functional and cosmetic changes?
+@item
+    Did you add tabs or trailing whitespace to the code? Both are forbidden.
 @item
     Is the patch attached to the email you send?
 @item