add pool fee explanation

[ppannuto]

when pool users are buying from the pool, they're charged fees proportional to the greatest debt of any pool member. While this is good from an incentive mechanism, we should add some text explaining what's happening

[westbl]

Is this still the way it currently works? Actually...does the Good Standing discount exist for pools? If yes, what are the criteria (like all members of the pool having >$20 in their balance, or only one having to have at least $20 as long as everyone is in the black)? If not, is the only discount that can be applied to pools a negative (based on greatest debt of any single pool member)? Does this show up at all (i.e., does the debtor fee show up if a pool user with a positive personal balance is using the pool to pay for a product when another pool user is in debt)?