Update README with Configuration Options

From Eppstein:

There's some things that are missing in this documentation so far, not sure if you have them as todo items or if they're accidentally omitted:

- preprocessor integration. I'd like to add a section on that. The nitty-gritty details may not best for this readme, but somewhere. My goal is to avoid any confusion about whether Sass is supported.
- compilation options. If not here, a link to it would be good.
Near the top where it talks about contributing, we need to mention our code of conduct.
- A section on how to get started using css-blocks (which probably points people to getting started guides for specific template/app integrations.

Start with the README, especially the contributing section, and inspect any existing getting-started or template/app integration guides. Document preprocessor integration, compilation options or a link to them, the code of conduct, and getting started guidance; done means each requested topic is covered without leaving Sass support unclear.

Written by the indexing model from the issue text.