'Building Docs' wouldn't work naturally as 'contributing' docs described

Hi, I'm a new contributor here, and I found that the `docs` section in CONTRIBUTING.md may not work.

### Current behavior
After running `npm install`, running `npm run docs:watch` would report error, as the screenshot:

### Expected behavior
Run gitbook in local environment as expected.

### Your environment
node v8.1.3, npm v6.2.0

I found the solution at issue in gitbook: https://github.com/GitbookIO/gitbook/issues/1818, after going into my local `~/.gitbook/versions/3.2.2` directory and run `npm install` command, things can work now.

That's tricky. It may be caused by the issue from gitbook, however this section of CONTRIBUTING docs just doesn't work as expected.

Start with the docs section in CONTRIBUTING.md and run npm install followed by npm run docs:watch to reproduce the reported GitBook failure. Compare the local GitBook version under ~/.gitbook/versions/3.2.2 with GitBook issue 1818; the work is done when the documented setup runs successfully in a fresh local environment.

Written by the indexing model from the issue text.