doc/developer: add url for sample files

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/developer.texi

@@ -543,6 +543,10 @@ tools/trasher, the noise bitstream filter, and
 should not crash, end in a (near) infinite loop, or allocate ridiculous
 amounts of memory when fed damaged data.
 
+@item
+Did you test your decoder or demuxer against sample files?
+Samples may be obtained at @url{http://samples.ffmpeg.org}.
+
 @item
 Does the patch not mix functional and cosmetic changes?