Update readme instructions for import PropTypes

[spchaplin]

Please update the following instructions in your readme:

import React, {Component, PropTypes} from 'react';

Because PropTypes is now a separate package, and not part of React anymore, I think this should read:

import React, {Component} from 'react';
import PropTypes from 'prop-types';

Otherwise, the user will get a build error that Proptypes.func is not defined.

You might also note that the user will need to install PropTypes with npm install prop-types

Thanks for your consideration!