Document how to use ssh-agent pull mode

[tombyman]

When trying to implement borg in our infrastructure I found out that there is no documentation for using borg in pull mode with ssh-agent for forwarding of the authentication information from server to client. From my point of view this method is more convenient than using socat because it have less prerequirements and overhead.

More detalied explanation in PR.

Have you checked borgbackup docs, FAQ, and open Github issues?

Yes

Is this a BUG / ISSUE report or a QUESTION?

ISSUE