Documentation of options in README

Hi.
Thanks for formage-admin.

I had to look around source a little bit to find how to tweak the admin the way I want it to behave.
The options are there, it's a pity they aren't documented, so here's my try at it.
I didn't document `model.formage.form` and `model.formage.post` because I'm not sure yet how it works...