Document alternative key types

[slifty]

When setting up with @meisekimiu she was facing issues with the key type that our document generates; it's not clear why, but we should document alternatives in the event there is an issue.

From the ssh2 documentation:

Default list (in order from most to least preferable):

    ssh-ed25519 (node v12.0.0+)
    ecdsa-sha2-nistp256
    ecdsa-sha2-nistp384
    ecdsa-sha2-nistp521
    rsa-sha2-512
    rsa-sha2-256
    ssh-rsa

She used rsa-sha2-512 and it worked just fine.