README updates

@kevinreedy @jdoconnor

- `direnv` is optional and not familiar to most
- no longer doing subdirectory variants
- `fig up` should be run detached
- clarify phrasing on `-advertise` IP
The auto-detected advertise IP for Consul-in-Docker on Boot2docker does not appear to be routable from the Nginx container. Use the following to look up the IP to set for `-advertise`: