Document PR process to establish and decommission testnets

[jevonearth]

As a protocol developer, I want to get a testnet running for my team's new protocol version so that a real-world publically accessible testnet can work allowing other teams to test against it and give me feedback and bug reports.

Proposed Solution

Define a process where a PR is created that adds the appropriate yaml that defines the new network, including docker image, constants and so forth.

The process to decommission a testnet must also be defined (should be as easy as removing the relevant testnet yaml declarations).