Add docs for using Promises and making AJAX requests

Created by: rogeliog

#521 (closed) This adds some simple documentation for the fetch and Promise polyfills.

@gaearon Should we also add a simple example of how to use this with React? Or do you think it falls out of scope?

class MyComponent extends Component {
  state = {}

  componentDidMount() {
    fetch('/todos.json')
    .then((response) => response.json())
    .then((todos) => this.setState({ todos: todos }))
    .catch((error) => console.log('Error', error));
  }

  render() {
    return <TodoList todos={this.state.todos} />;
  }
}