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](facebook#6423 (comment)), 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 facebook#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 facebook#6510

Differential Revision: D3070560

Pulled By: vjeux

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

CONTRIBUTING.md

@@ -19,11 +19,17 @@ The core team will be monitoring for pull requests. When we get one, we'll run s
 1. Fork the repo and create your branch from `master`.
 2. **Describe your test plan in your commit.** If you've added code that should be tested, add tests!
 3. If you've changed APIs, update the documentation.
-4. Add the copyright notice to the top of any new files you've added.
-5. Ensure tests pass on Travis and Circle CI.
-6. Make sure your code lints (`node linter.js <files touched>`).
-7. Squash your commits (`git rebase -i`).
-8. If you haven't already, sign the [CLA](https://code.facebook.com/cla).
+4. If you've updated the docs, verify the website locally and submit screenshots if applicable
+```
+$ cd website
+$ npm install && npm start
+go to: http://localhost:8079/react-native/index.html
+```
+5. Add the copyright notice to the top of any new files you've added.
+6. Ensure tests pass on Travis and Circle CI.
+7. Make sure your code lints (`node linter.js <files touched>`).
+8. Squash your commits (`git rebase -i`).
+9. If you haven't already, sign the [CLA](https://code.facebook.com/cla).
 
 #### Copyright Notice for files