add instructions for testing website locally

Summary:Explain the **motivation** for making this change. What existing problem does the pull request solve?

> When vjeux [asked me how the documentation changes looked](https://github.com/facebook/react-native/pull/6423#issuecomment-195608638), it wasn't obvious to me how to build and view them locally. So, I thought it might be a good idea to add instructions to the contributing document.

I was able to run the website locally when working on PR #6423 but am no longer able to now. I have since updated my local repo with a `git pull`. I've deleted the node_modules directories in both the root and website folders, rebuilding both. Still, I get the following error when I run `npm start` within the website directory (problem with the ES2015 compiling?)...

```bash
**/react-native/website/server/convert.js:134
    .filter(key => key.startsWith('RN_'))
                ^^
SyntaxError: Unexpected token =>
    at exports.runInThisContext (vm.js:73:16)
    at Module._compile (
Closes https://github.com/facebook/react-native/pull/6510

Differential Revision: D3070560

Pulled By: vjeux

fb-gh-sync-id: 6d200bb545d0ce357500cd666dd400ec82fafa92
shipit-source-id: 6d200bb545d0ce357500cd666dd400ec82fafa92