Add documentation

Adds full documentation (migrated from the wiki) to the project. Addresses #21.

**Question:** I have links to the Configuration and Layouts pages in the README. Where should I point them to?
Would be great here to have a comment that says even `# _layouts/archive.html` and `# _layouts/custom-archive-layout.html` so they know immediately where they can expect to find the file for the setting they choose.