Refine docs

Aligning with: https://github.com/thoughtbot/bourbon/pull/578

- Add “quick links” near top of README to docs, changelog and issues
- Tweak wording for installation
- Remove CLI section in favor of just pointing people to the wiki page
- Point Bourbon family links to GitHub repos instead of marketing sites
- Use new thoughtbot logo (SVG)
- Fixes #224
Moved to [Line 15](https://github.com/thoughtbot/neat/pull/269/files#diff-04c6e90faac2675aa89e2176d2eec7d8R15)