Document requirement for running ssh-agent

Currently documentation states: "Nothing else than docker being installed on your host. You don't need any other tool - none!". However ssh-agent must run in order to avoid mount error:

"Error response from daemon: OCI runtime create failed: invalid mount {Destination::/ssh-agent Type:bind Source:/var/lib/docker/volumes/.../_data Options:[rbind]}: mount destination :/ssh-agent not absolute: unknown".

Also maybe document in this context l3d reset to recover after error.