Merge pull request #17332 from twbs/themes-contact

Mention themes contact info in CONTRIBUTING.md

CONTRIBUTING.md

@@ -28,6 +28,8 @@ restrictions:
   [`Normalize`](https://github.com/necolas/normalize.css) (open them in
   their respective repositories).
 
+* Please **do not** open issues regarding the official themes offered on <http://themes.getbootstrap.com/>. Instead, please email any questions or feedback regarding those themes to `themes AT getbootstrap DOT com`.
+
 
 ## Issues and labels