Update README.md

Make it explicit that the 'installing as a plugin' section is assuming that you've installed pyenv in the standard location and what to do if you've done something else.  Clarify the language a little bit about checking out a specific release vs. pulling the current development version.
Update 'Using `pyenv virtualenv` with pyenv' section.

Explicitly define the parameters that you pass into 'pyenv virtualenv' and what they stand for.
Merge pull request #3 from eikonomega/patch-1

Update README.md