Add a screenshot to README

Screenshots help people see what the project is about.

![2013-04-28_17 50 33](https://f.cloud.github.com/assets/180404/436507/c5ae7eea-b051-11e2-805d-abd1a76ee347.png)

The screenshot is of a path being walked. This is a screenshot of the latest build from master (as of writing).