modification to developer install steps

[dbfreem]

I was trying to follow the developer install steps on the readthedocs site. Two observations

1. Shouldn't the steps tell the user to activate their virtualenv before running python setup.py develop. Like a source bin/activate or bin/activate

2. Not sure if this one matters or not but noticed the repo that it says to clone is at onyxfish/agate and not wireservice/agate

I can submit a PR for these if you like but wanted to put in an issue first.

[jpmckinney]

When using mkvirtualenv from virtualenvwrapper, the venv is automatically activated. That said, we'd accept a PR that used the plain virtualenv package instead.