change instructions to allow connection to a headscale server

[advisorgee]

Can the readme be updated to include the option to use a custom login server (headscale)?

tailscale up --login-server=https://server.example.com:8080 --advertise-routes 192.0.2.0/24 --advertise-exit-node --authkey tskey-XXX

[jamesog]

There isn't any value in enumerating all possible options to tailscale up. People should be referring to the official Tailscale docs for how to use the CLI tools. The section that's there does say that it's an example.