Add IdentitiesOnly to ssh config in README

This is useful when you use yubikeys, as otherwise the ssh-agent will
offer your secure key on each Ansible task.

Author: MoteHue

README.rst

@@ -212,6 +212,7 @@ If the deployed instances are behind an SSH bastion you must ensure that your SS
       IdentityFile ~/.ssh/key
       UserKnownHostsFile /dev/null
       StrictHostKeyChecking no
+      IdentitiesOnly yes
 
 Configure Terraform variables
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