add details on backing up your SSH keys

Users need to backup SSH keys if they want to do a complete restore to the same domain and not have users get the `WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!` and man-in-the-middle attack warning messages. Related to https://github.com/gitlabhq/gitlabhq/pull/8359.
SSH keys -> SSH host keys
seperate -> separate
Merge pull request #5 from bbodenmiller/patch-1

add details on backing up your SSH host keys