Document wallet feature data entry process

[polymutex]

Prior to launch, we would like to make it easy for wallet development teams to tell us about their wallet. The current process requires a PR, and has many pitfalls due to the complex featureset that Walletbeat looks for.

We should try to ease this pain as much as possible. At minimum, we should have Markdown-type documentation to go through the process of populating wallet feature data. In the longer term, this should be more automatable.