Related to #432: Add instructions to Contribute docs on how to install requirements

I added an instruction on how to install the requirements listed in requirements.txt and dev_requirements.txt. This seemed like a good idea, because it wasn't obvious to me at the beginning. There are some instructions on this subject in the Quick Start guide (http://freeseer.readthedocs.org/en/latest/quick-start.html) but they seem to be aimed at people installing Freeseer, rather than running it from the source. #432 seemed like the most relevant pre-existing issue to match this change.