Note ticket and source version for feature3951 in changes file

changes/dirreq-stats-default

@@ -1,4 +1,5 @@
   o Minor features:
     - Turn on directory request statistics by default and include them in
       extra-info descriptors. Don't break if we have no GeoIP database.
+      Backported from 0.2.3.1-alpha; implements ticket 3951.