Added docs for find-occurrences

This pull-request should be merged whenever we release Scala Search 0.1 :)

When rendered it looks like this:

![screenshot on 2013-05-30 at 15 20 21](https://f.cloud.github.com/assets/83561/584989/71a5422c-c92c-11e2-88a4-a44315d841fa.png)