added a few lines

To help myself and perhaps other linux know-nothings work through this. Still stuck on getting the `slatkin-exact-tools` module installed though. I found that I had to `sudo apt-get build-dep matplotlib` to make the `pip install -r requirements.txt` line work. Perhaps because Lubuntu is a slimmed down version of Ubuntu?
Update README.md
Merge pull request #1 from benmarwick/patch-1

added a few lines