An ABE usage presentation

@nwhite89 this is the presentation I am working on. In fact I did one session on a plane and it's been taking a rest since then. But I'd like to turn this into a detailed case of using ABE in practice, maybe we can split the work of adding to it.

Not sure this repo and this folder are the right places for it, but at least it's a starting point of a discussion of a more general ABE general documentation and potentially an ABE site.
Sounds good. I've tweaked to get rid of the passive: `this would not encourage us to design an API before building ...`