Add readme

Status: **Opened for visibility**
Reviewers: @ry5n @jeffkamo 

## Changes
- Added a readme

## Notes
- Not sure if we really need as much detail about the variables file as I've added here. Might be worth dropping that section. Let me know your thoughts.
- Fixes #3 

We can probably avoid the whole qualifier/property/application concept. In reality we have variables (named after css properties where applicable) and qualifiers prepended. When and if we use these terms, we should explain them first. We can probably remove points 2 and 3, and move point 5 above point 4.

* If variables map closely to a css property, e.g. font-family, use this property in the name, e.g. `$small-font-family`.
* Variables are often part of a set; use a *qualifier* prefix to distinguish them, e.g. `$small-font-size`, `$form-font-size`.
* Variables without a qualifier are considered “base” variables, i.e. the default value for that type, e.g. `$font-size`, `$brand-color`, `$grey-50`