New SASS guide

More than happy to hear about...

* Stuff I've missed
* Bad examples
* Poor English
* Problems with structure
* My awesomeness :?:
* ...
* Just plain questions

I expect to follow this up as the component library/styleguide becomes clearer. And as we roll out some KSS into the new Rails happy path.

Expect a presentation version of this too in short(ish) order.
Similar to specifying `line-height` in `em`?