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.

Fixed trailing whitespace and re-submitted. Thanks @rorcores 

> and re-submitted.

For the future, if you add another commit and push it, the pull request will be updated, there's no need to open a new one > Thanks @rorcores
> 
> > and re-submitted.
> 
> For the future, if you add another commit and push it, the pull request will be updated, there's no need to open a new one

Thanks @MarcoGorelli, really good to know. @datapythonista care to look? Thanks @rorcores 