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 

Should these be inline code of the actual variables names (i.e. `$backdrop-layer`) instead of quoted text?