Needs docs on installing engines

[tiye]

It actually took me a long time to setup environment on Ubuntu, and for OS X I failed. As I can recall from my Bash history, I tried to install these packages with apt-get:

• build-essential
• `libcairo-dev
• graphicsmagick (not sure)
• imagemagick

and some modules from npm:

• canvas
• gm
• node-gyp
• phantomjs

before I installed this plugin,

And for OS X, I failed to make cairo available, so did the things for canvas or phantomjs.

I think we need docs here so that people pick up the engines more quickly.

[twolfson]

There is a requirements section with either inline instructions or points where to go for better instructions. Closing issue.

https://github.com/Ensighten/grunt-spritesmith#requirements