readme: SETTING_EXTERNAL_HOST needs to contain a top level domain

[karlicoss]

I am setting up a Zulip instance in my home LAN, so at first tried setting SETTING_EXTERNAL_HOST: "mypchostname:1234".

However this resulted in an Internal Server Error page -- after inspecting the logs, it was due to this line https://github.com/zulip/zulip/blob/81bd63cb46273b8c94ef9e92c00893ed97110119/zerver/models.py#L4904

Seems that FAKE_DOMAIN_NAME isn't exposed for the docker setup, so I decided the easiest option for me was just to add an alias like mypchostname.lan in the hosts file.

But it's a bit of a gotcha -- not sure whether it requires a proper fix, or worth exposing that FAKE_DOMAIN_NAME setting -- so figured at least mention in the docs might help some people.