Merge pull request #19470 from twbs/plus-1-stfu

CONTRIBUTING.md: Prohibit "+1" comments in favor of reactions
[ci skip]

CONTRIBUTING.md

@@ -24,10 +24,16 @@ restrictions:
 * Please **do not** derail or troll issues. Keep the discussion on topic and
   respect the opinions of others.
 
+* Please **do not** post comments consisting solely of "+1" or ":thumbsup:".
+  Use [GitHub's "reactions" feature](https://github.com/blog/2119-add-reactions-to-pull-requests-issues-and-comments) 
+  instead. We reserve the right to delete comments which violate this rule.
+
 * Please **do not** open issues or pull requests regarding the code in
   [`Normalize`](https://github.com/necolas/normalize.css) (open them in
   its repository).
-* 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`.
+
+* 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