Update docs for Babel 6

[justingreenberg]

@thejameskyle can this line be removed?

Note that when using React.createClass() and allowing babel to extract the displayName property you must ensure that babel-plugin-react-display-name is included before react-transform. See this github issue for more details.

[jamiebuilds]

We can only remove that if https://github.com/babel/babel/tree/master/packages/babel-plugin-transform-react-display-name adds support for:

var Foo = bar(React.createClass({
}));