Document how to specify route as URI

[rocketraman]

How can I specify my route as a URI rather than mounting a volume?

[rocketraman]

It looks like one can use the ROUTES_URI env variable like this:

  --env ROUTE_URIS=kafka://kafka-1:9092?topic=logs,kafka://kafka-2:9092?topic=logs,kafka://kafka-3:9092?topic=logs \

but it would be useful to have this in the readme.