Update README instructions

[monfresh]

I had trouble running grunt watch on my machine. I had to install the CLI tool to get it to work, per the Grunt Getting Started guide:

npm install -g grunt-cli

Also, the README should point out that Node.js is a prerequisite.

[anselmbradford]

For reference, I opened an issue to create a how-to for this at codeforamerica/howto#14, but perhaps it's too simple to have its own.