Fix prefer-stateless-function (fixes #588)

Author: yannickcr

docs/rules/prefer-stateless-function.md

@@ -8,8 +8,8 @@ This rule will check your class based React components for
 
 * methods/properties other than `displayName`, `propTypes`, `render` and useless constructor (same detection as ESLint [no-useless-constructor rule](http://eslint.org/docs/rules/no-useless-constructor))
 * instance property other than `this.props` and `this.context`
-* `render` method that return anything but JSX (`undefined`, `null`, etc.)
 * presence of `ref` attribute in JSX
+* `render` method that return anything but JSX: `undefined`, `null`, etc. (only in React <15.0.0, see [shared settings](https://github.com/yannickcr/eslint-plugin-react/blob/master/README.md#configuration) for React version configuration)
 
 If none of these 4 elements are found, the rule will warn you to write this component as a pure function.
 
@@ -23,14 +23,16 @@ var Hello = React.createClass({
 });
 ```
 
-The following patterns are not considered warnings:
+The following pattern is not considered warnings:
 
 ```js
 const Foo = function(props) {
   return <div>{props.foo}</div>;
 };
 ```
 
+The following pattern is not considered warning in React <15.0.0:
+
 ```js
 class Foo extends React.Component {
   render() {