Adds FAQ about why payments can decrease

This plainly establishes the mechanisms by which donations can decrease.

The aim is to eliminate accusations that anyone involved in the Gratipay
machine can wantonly interfere with someone else's donations unchecked
without prior notice, where unprohibited by law.
We don't necessarily have good contact info for everyone. We would of course make a good-faith effort to contact anyone that we were planning to ban (we would first need to build a banning mechanism, of course). I guess we're okay to leave it unspecified here how exactly we would contact someone we are banning?