Port twbs#19470 to v3

CONTRIBUTING.md: Prohibit "+1" comments in favor of reactions

Refs https://github.com/blog/2119-add-reactions-to-pull-requests-issues-and-comments
Refs dear-github/dear-github#141 (comment)
[ci skip]

Author: cvrebert

CONTRIBUTING.md

@@ -24,11 +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
   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`.
+* 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`.
 
 
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