This is from the original documentation, which says to use pip-2.7 (see the diff). I'm merely updating it to suggest installing from `requirements.txt` instead of installing sphinx directly since that is the standard.

Just to be clear, do you want me to amend the PR to:

* excise all mentions of pip-2.7
* suggest using virtualenv, eliminating the need for `sudo` system-wide installs

Is that accurate?