Add "Did you provide a clear git commit log message?" to the patch checklist

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/developer.texi

@@ -326,6 +326,8 @@ send a reminder by email. Your patch should eventually be dealt with.
     Did you sign off your patch? (git commit -s)
     See @url{http://kerneltrap.org/files/Jeremy/DCO.txt} for the meaning
     of sign off.
+@item
+    Did you provide a clear git commit log message?
 @item
     Is the patch against latest FFmpeg git master branch?
 @item