Add an FAQ (perhaps as a wiki page?)

I'm sure I'm not the only one who is wondering "Why not put the proof in the image metadata?" and "Okay, I grant that you can't have the signature be part of the thing signed, but you could just have the final signature mean 'the signature of this file when this signature is removed', or am I missing some nuance somewhere?"

It might save some work to have a place for those answers to live.