Add contributor guidance to payment methods doc

[m52go]

Upcoming payment methods doc will have details on payment method parameters (maximum trade amounts, trading periods, etc) and account signing for users, but it will lack contributor guidance on the following topics:

• How we think about adding payment methods, payment method hardness scale

• Payment method roles

• How to suggest a new payment method (i.e., issue template)

[pazza83]

@m52go Just commenting here so I pick this up :)

[m52go]

Excellent! Note that docs aren't being updated as the wiki is succeeding it. There's an article for payment methods already there...perhaps you could add to it. Please DM @wiz for an account if you don't already have one.