Update doc regarding webpack and React 15

[tylercollier]

I've updated doc regarding webpack and React 15, which came from the advice here.

What I had to do to get things working with webpack and React 0.14 was different than in the guide, and I wrote about it here. I did not include it in this PR due to not really understanding what's going on and if my solution is a universal one, but I think a better guide will make sure people don't abandon using enzyme because they feel it's too hard to set up.

[lelandrichardson]

LGTM. We added react 15 support, but didn't really update the documentation. Thanks for the help!

[tylercollier]

Can you update the doc for React 0.14 too?

The current guide says:

If you are using React 0.14, the instructions above will be the same but with a different list of externals:

In its list that follows, it isn't as helpful as what's shown for using with React 0.13 (which is actual javascript). I recommend writing actual javascript because with just a list, it's unclear what the values should be ('window' or true).

So something like this:

/* webpack.config.js */
// ...
externals: {
  'cheerio': 'window',
  'react-dom': ???
  'react/lib/ExecutionEnvironment': true,
  'react/lib/ReactContext': true
}
// ...

Note the ???. For me, to get things working, I actually removed that line and replaced it with 'react-deom/server': 'window'. I'm not sure how 'correct' that is vs just my environment.