Update react.md to use imported Component

[joshwcomeau]

I believe it's safe to say, after the internal vote, that people prefer:

import React, {Component} from "react";

class Thing extends Component { ... }

over

import React from "react";

class Thing extends React.Component { ... }

The examples in react.md all use the React.Component style, so we should update them to just extend Component. We can also add a blurb about the import convention?

cc @kevinbarabash