Document requirements of traefik and etcd

[minrk]

As we get traefik with etcd working, it's time to start thinking about how this is going to work in production.

When we deploy this in the helm chart, for example, etcd and traefik are going to be run externally, not started by our code here. For these reasons, it's going to be important to enumerate exactly what our requirements are for how traefik and etcd are configured, and what the Python code here needs to be told about.

Things we probably need:

1. do we require the traefik api to be enabled for waiting for routes? If so, we need a url and authentication info for the api
2. etcd url and credentials
3. what configuration does traefik need to be started with (e.g. enable api, etcd storage, etc.)?
4. etcd prefixes? Are these dictated by us, or are they given by users?

[minrk]

@GeorgianaElena I think this should be the current task. Let's try to describe what's needed for deploying and configuring traefik-proxy, which should help with jupyterhub/the-littlest-jupyterhub#266 .

I added some todo items in the docs in #36