Document API Practices Options

[Popsiclestick]

I thought it might be worth adding some documentation around the API defaults options (https://github.com/Netflix/stethoscope/blob/master/stethoscope/api/defaults.py#L135)

It's not immediately clear what key options are available, what their possible values are, what that might do, and how that data is fed throughout Stethoscope.

[andrewmwhite]

There's some initial work on that documentation here, but in writing it I realized how unhappy I was with the way the system currently works. Therefore, it's subject to change while we get our ducks in a row.