Documentation to run a relay "out of the box"

[ralexstokes]

It would be nice if a user could clone the repo and run the relay with just a few steps (documenting anything outside the basic workflow).

When trying to do this myself, I've run into a few issues:

1. Config file
   The relay expects a config file with a somewhat complex set of options. It would be nice if an example config.yml was present at the repo root so that something like cargo run just works.

2. Documentation in the readme on external services
   It looks like the config expects references to the following external systems:
   a. Postgres
   b. Redis
   c. "simulator" -- payload validator I think at the moment
   d. beacon client with HTTP API enabled

It would be nice if the README called these out explicitly so a user can have them running before trying to run the relay (and presumably failing).