Document need for env_keep+=SSH_AUTH_SOCK

[Matthijs Kooijman]

I just tried installing the Artful version (0.10.3-1) on a Xenial install, and had some problems making it work. It seems that I needed `Defaults env_keep+=SSH_AUTH_SOCK` (or, actually, I used `Defaults env_keep+=SSH_*`) to let pam-ssh-agent-auth see the SSH agent and connect to it. Perhaps a note about this in the README would be helpful, if this is really always required? Or is this not needed with newer sudo versions (I'm using 1.8.16).

On a related note, I had some problems with directory permissions (this Ubuntu install had group- and world-write bits enabled on / and /etc). This is probably not a problem on a default install, but it might be good to mention these restrictions when someone wants to move the key file somewhere else (though a proper error message is logged to auth.log by default, even without debug, so that's at least helpful).