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?
I have a feeling people who only want one option will say

```yaml
enabled: year
```

and it will bork. Maybe one example of where you only have one option turned on to show that `all` is the only accepted `String` value?