Added design doc.
Added design doc.

Summary:
Add much more comprehensive backend design doc in a place that other people stumbling ont he project will be able to see, organized more thoroughly than the doc we have now, with more design decisions made.

Please pick this apart, because I'm going to use it to close T17.

Fixes T17.

Reviewers: applmak

Reviewed By: applmak

CC: jwall

Maniphest Tasks: T17

Differential Revision: http://d.sil.as/D39

bsimser/rise-to-power,bsimser/rise-to-power,bsimser/rise-to-power,bsimser/rise-to-power,bsimser/rise-to-power