ref CSSLint suggestion

Author: cvrebert

README.md

@@ -111,5 +111,6 @@ The following incompatibilities have been successfully documented in [MDN](https
   * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input$compare?to=588351&from=588303
 * [`resize` requires `overflow` other than `visible`](https://github.com/twbs/bootstrap/pull/13600)
   * [Is already documented in MDN.](https://developer.mozilla.org/en-US/docs/Web/CSS/resize) The Bootstrap source code comment was in error.
+  * Also filed https://github.com/CSSLint/csslint/issues/517
 
 Hopefully these edits will survive in some form and not get wholesale reverted.