Add info on using virtual environments.

Include a couple of virtual environment options.
Remind users to activate environment prior to installing
requirements via pip.
Merge branch 'master' of https://github.com/eikonomega/XBlock

# By Michael Dunn
# Via Michael Dunn
* 'master' of https://github.com/eikonomega/XBlock:
  Remove extraneous text.
  Incorporating feedback received.