PyCon 2014 Sprint Additions

1. Add 'Required Background Knowledge' section
2. Add instructions on using virtual environments
3. Minor syntax updates
Add reference to CONTRIBUTING.rst in edx-platform repo.
Add Mike Dunn <mike@eikonomega.com>
Chose one way of creating virtual environments.
Merge pull request #204 from eikonomega/master

XBlock Documentation Update: PyCon 2014