Update README.md

Link in to the custom operations wiki page.

Author: jwadhams

README.md

@@ -125,3 +125,9 @@ curl -O https://raw.githubusercontent.com/jwadhams/json-logic-js/master/logic.js
 This library makes use of `Array.map` and `Array.reduce`, so it's not *exactly* Internet Explorer 8 friendly.
 
 If you want to use JsonLogic *and* support deprecated browsers, you could easily use [BabelJS's polyfill](https://babeljs.io/docs/usage/polyfill/) or directly incorporate the polyfills documented on MDN for [map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) and [reduce](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce). 
+
+## Customization
+
+It's not possible to include everyone's excellent ideas without the core library bloating, bringing in a ton of outside dependencies, or occasionally causing use case conflicts (some people need to safely execute untrusted rules, some people need to change outside state).
+
+Check out the [documentation for adding custom operations](http://jsonlogic.com/add_operation.html) and be sure to stop by the [Wiki page of custom operations](https://github.com/jwadhams/json-logic-js/wiki/Custom-Operations) to see if someone has already solved your problem or to share your solution.