Add ability to customize the ready screen on empty admins

[fzaninotto]

import * as React from 'react';
import { Admin } from 'react-admin';

const Ready = () => (
    <div>
        <h1>Admin ready</h1>
        <p>You can now add resources</p>
    </div>
)

const App = () => (
    <Admin ready={Ready}>
        ...
    </Admin>
);

This will help build an offline-first distribution of react-admin.

[Luwangel]

Just one last review about the documentation and I'll merge it :)