Rewrite build system documentation to document new npm scripts.

Created by: akwick

Things done:

• cloned bootstrap (v4-dev latest commit was: fb3303f5)
• $npm install
• $npm test finishes and succeeds

When running grunt test-js:

bootstrap]$ grunt test-js
Warning: Task "test-js" not found. Use --force to continue.

Aborted due to warnings.

Running only grunt yields to the same result.

Changing to branch master, grunt test-js runs.