DOC: Added documentation for building using pyenv
Added documentation for building using pyenv. Discussed this PR with @WillAyd on gitter on May 20.

Also fixed up another line he mentioned (python setup.py build_ext --inplace -j 4
instead of:
python setup.py build_ext --inplace -j 0) for conda/venv/pyenv.

Have rebuilt the docs and ensured they build properly.
