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.
Either: 'As Gratipay gifts are anonymous, we cannot tell you who.', or 'Gratipay gifts are anonymous, so we cannot tell you who.'