Document installation and use of Grunt for Largo development

[nrrb]

Grunt is used to run a few different tasks within Largo and there is no documentation on how to install it. For example, docs/readme.md explains how to use a grunt command to compile the RST docs into HTML.

@benlk explained to me that I needed to run npm install from the root of the Largo project folder, and after doing that I was able to use grunt for generating the documentation HTML. This instruction for npm install should appear somewhere and possible prerequisites for being able to use npm.

The tasks that Grunt performs should also be documented. @benlk said this includes:

• recompiling the LESS to CSS
• minifying the CSS
• uglifying the JS

[benlk]

Partially closed with #1036, for installing NPM. Still needs docs on what our Grunt tasks are and what they do.

[benlk]

• list of grunt tasks