Clarify environment variables used in README.md

[linux4life798]

It is unclear what actual environment variables are required and consumed by service binary and/or the docker container.

• The readme mentions TS_AUTH_KEY and TS_AUTHKEY. Which one is it?
• Do we need TSNET_FORCE_LOGIN? What does that variable do?
• What port related environment variables / arguments are required to be configured?

[mostlygeek]

The environment vars and docs for them was cleaned up a bit in #56.
Let us know if there are things that can be clarified a bit better

[linux4life798]

Awesome, thanks for cleanup and improvements! There is still a bit of ambiguity about how to authenticate this from a, zero TS-state, initial state. I added two comments to the #56 PR code, just as suggestions.