Document first time connection warning in setup

```
$ ssh -T git@github.com
The authenticity of host 'github.com (20.26.156.215)' can't be established.
ED25519 key fingerprint is SHA256:+DiY3wvvV6TuJJhbpZisF/zLDA0zPMSvHdkr4UvCOqU.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])?
```

