Review developer docs

[myersjustinc]

@ssempervirens was trying to get her local environment set up to work on associatedpress/datakit-data#25, and she ran into a few problems with the assumptions made in datakit-data's developer docs, including the use of pyenv-virtualenv and virtualenvwrapper.

Let's review those docs—and similar ones in other plugins—for consistency with one another, for accuracy and for ease of use.