Write philosophical guidance on adding an ExchangeAdapter

[yerdua]

Current Behavior:
Any exchange listing CELO against fiat currency has an adapter

Intended Behavior:
There should be some forward looking guidance for when cLabs or perhaps someone in the open source community can add an exchange adapter and when there isn't enough value to add it. As repo maintainers we should have a guide on what type of exchange adapter we are approving even if submitted by an anonymous contributor.