Clarify dependency installation instructions in README

After cloning the repository, I could not run npm because node/npm was not installed.

I recommend adding an instruction like this at the beginning of the README:

> 0) To install dependencies, you'll first need to have node and npm installed.  See [Downloading and installing Node.js and npm](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm) for guidance.
