Private key cannot have a passphrase

[felipecsl]

If the key generated with ssh-keygen has a passphrase set, later running ssh-add will require the passphrase with a terminal prompt which cannot be fulfilled during the workflow run, causing it to get stuck indefinitely. I could not find a way to provide the passphrase to ssh-add via command line so, to avoid confusion, I'd suggest just adding that as a disclaimer to the project readme, so people don't get tripped on that in the future. Thoughts?