rename entry.js to index.js

this is a remnant from the webpack tutorial. i think index.js sounds
nicer, even though it should get split up into multiple files and be
just setup at some point.

js/react/README.md

# visit index.html in your browser
$ vi entry.js      # change something!
$ vi index.js      # change something!
```
## Ideas

js/react/webpack.config.js

module.exports = {
	entry: "./entry.js",
	entry: "./index.js",
	output: {
		path: __dirname,
		filename: "bundle.js"