Relayer docs

[R0bi7]

Describe the issue
Because Relayer is an essential component of xCall and associated BTP, developers must be well-versed in it.

On top of that, many developers are probably wondering if Relayer would be a profitable business considering that it will generate revenue.

Thus I've raised this issue in order to incentivise core team working on Relayer to release public documentation answering at least before raised concerns.

Thank you.