readme update for #505

README.md

@@ -370,6 +370,8 @@ The display order of fragments can be controlled using the ```data-fragment-inde
 
 When a slide fragment is either shown or hidden reveal.js will dispatch an event.
 
+Some libraries, like MathJax (see #505), get confused by the initially hidden fragment elements. Often times this can be fixed by calling their update or render function from this callback.
+
 ```javascript
 Reveal.addEventListener( 'fragmentshown', function( event ) {
 	// event.fragment = the fragment DOM element