Document ssh-add error

README.md

@@ -1221,6 +1221,8 @@ The Yubikey will blink when it's waiting for the touch.
 
 - If you receive the error, `sign_and_send_pubkey: signing failed: agent refused operation` - you probably have ssh-agent running.  Make sure you replaced ssh-agent with gpg-agent as noted above.
 
+- If you receive the error, `Error connecting to agent: No such file or directory` from `ssh-add -L`, the UNIX file socket that the agent uses for communication with other processes may not be set up correctly. On Debian, try `export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"`
+
 - If you totally screw up, you can [reset the card](https://developers.yubico.com/ykneo-openpgp/ResetApplet.html).
 
 ## Yubikey OTP Mode and cccccccc....