Add a quick start section

[kaelig]

We usually don't describe the process to use modules and refer people to Origami's documentation, but o-grid is so ubiquitous, people are likely to start using Origami from there. In that sense, it feels right to make it really easy.

[kaelig]

Alongside this PR, I've registered o-grid on the default bower registry so that anybody can use it using these same steps.

[AlbertoElias]

:ship: it