Add description for feature_request/open/approved.

patch by Mark Cox, melbournemark+ffmpeg gmail com

Originally committed as revision 10403 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/issue_tracker.txt

@@ -162,6 +162,10 @@ patch/closed/applied
 patch/closed/rejected
     Patches which have been rejected.
 
+feature_request/open/approved
+    Feature requests which FFmpeg developers agree are likely to be
+    accepted if a good quality patch is submitted.
+
 feature_request/open/needs_more_info
     Feature requests where its not clear what exactly is wanted
     (these also could be closed as invalid ...).