Mention patcheck.

Patch by Zhihang Wang, zhihang D wang A gmail

Originally committed as revision 20780 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/developer.texi

@@ -252,6 +252,9 @@ keeping it as a logical unit that contains an individual change, even
 if it spans multiple files. This makes reviewing your patches much easier
 for us and greatly increases your chances of getting your patch applied.
 
+Use the patcheck tool of ffmpeg to check your patch.
+The tool is located in the tool directory.
+
 Run the regression tests before submitting a patch so that you can
 verify that there are no big problems.