document how to use rcov
Created by: charlietanksley
I'm a little uncertain about this one. I didn't know if it needed associated features or if a README change was sufficient, and I didn't know if all these methods should be documented (or if there were others?), but I thought I needed to start somewhere. Let me know what needs to change and I'm happy to revise.
Closes #479 (closed) Closes #551 (closed)