Document how to use OS X KeyChain for Passphrase

[keithy]

Just a suggestion

The following explains https://www.netmeister.org/blog/keychain-passwords.html

security add-generic-password -a ${USER} -s borgbackup-repo -w PassPhraseYouWillNeverGuess

in .bashrc (or similar)

export="BORG_PASSCOMMAND="security find-generic-password -a ${USER} -s borgbackup-repo -w"

To use this you likely need the -E option for sudo

alias borg-list='sudo -E borg list /Volumes/Repo/repo'