Add description for feature_request/open/approved.

patch by Mark Cox, melbournemark+ffmpeg gmail com


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@10403 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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 ...).