Improve the organization of the docs -- put important things in README

People are looking for usage docs in the README, and that's where they should be. I've seen too many "I can't run rake convert" questions over and over. This will solve that and help so many people.

Also added the word "Compass" all over the compass section to reduce confusion for Rails people.

Fixes #434 

**Note:** There's a big change in here, which is that I added docs about Bootstrap 3 and how to get it to the Installation section. I can move this just down to the Development section where people find it usually, or I can just remove the reference altogether. But I think people will ask for it and want to try it out, so it should be good.

[Preview the file in this branch here.](https://github.com/thomas-mcdonald/bootstrap-sass/blob/trisweb-update-docs-to-be-clearer/README.md)