add instructions to README.md, include `npm install` in `make build`.

Summary:
make build fails when running the first time, which is a pain because it gives absolutely no indication of why it failed except that it can't find browserify.

this allows make build to run correctly. also it adds some more documentation.

Test Plan: run `make build` after `git p` succeeds

Reviewers: eater, jack

Reviewed By: jack

Subscribers: jack

Differential Revision: http://phabricator.khanacademy.org/D9402