Added documentation for babel transform plugin

First time users who don't understand the structure of the gem have no way of knowing how to add transform plugins, or what version of babel react-rails is using to use the correct syntax

Author: G-Simpson

README.md

@@ -199,6 +199,18 @@ Components must be accessible from the top level, but they may be namespaced, fo
 
 `react-rails` provides two transformers, `React::JSX::BabelTransformer` (which uses [ruby-babel-transpiler](https://github.com/babel/ruby-babel-transpiler)) and `React::JSX::JSXTransformer` (which uses the deprecated `JSXTransformer.js`).
 
+#### Transform Plugin Options
+
+To supply additional transform plugins to your JSX Transformer, assign them to `config.react.jsx_transform_options`
+
+`react-rails` uses the Babel version of the `babel-source` gem.
+
+For example, to use `babel-plugin-transform-class-properties` :
+
+    config.react.jsx_transform_options = {
+      optional: ['es7.classProperties']
+    }
+
 ### React.js versions
 
 `//= require react` brings `React` into your project.