[readme] Clean up isMounted section in react readme

When I added this section in f2dc504 I didn't do a very good job of
following the style used in other parts of this repo. This commit
remedies that.

Author: lencioni

react/README.md

@@ -337,7 +337,9 @@
 
 ## `isMounted`
 
-  [`isMounted` is an anti-pattern][anti-pattern], is not available when using ES6 classes, and is on its way to being officially deprecated. Its use should be avoided.
+  - Do not use `isMounted`.
+
+  > Why? [`isMounted` is an anti-pattern][anti-pattern], is not available when using ES6 classes, and is on its way to being officially deprecated.
 
   [anti-pattern]: https://facebook.github.io/react/blog/2015/12/16/ismounted-antipattern.html