More detailed docs on community credit use case

[mDuo13]

The community credit use case is a place the current docs are a little lacking in describing what the XRPL is really capable of. In particular, some diagrams could really help to show how it works.

For some ideas, check out the "Beyond banking" section of this old Ripple wiki article (Note: "Ripple" mostly refers to the XRP Ledger, not the company, in that article.)

Or, here's the start of another, similar diagram [warning, there may be typos here].

Note, some conventions from these types of diagrams that are useful to understand and maybe preserve:

• Circles generally represent "accounts" while lines represent trust lines
• Arrows on trust lines represent the direction of trust. In the case of a stablecoin, this means that "the arrows point to where the money is" (that is, the real assets that back the stablecoin).