Add ssh troubleshooting to readme

[hackartisan]

misleading error message looks like

net-ssh requires the following gems for ed25519 support:
 * rbnacl (>= 3.2, < 5.0)
 * rbnacl-libsodium, if your system doesn't have libsodium installed.
 * bcrypt_pbkdf (>= 1.0, < 2.0)
See https://github.com/net-ssh/net-ssh/issues/478 for more information
Gem::LoadError : "can't activate rbnacl (>= 3.2.0, < 5.0), already activated rbnacl-7.1.1. Make sure all dependencies are added to Gemfile."
(Backtrace restricted to imported tasks)
cap aborted!
NotImplementedError: OpenSSH keys only supported if ED25519 is available

If you are having the above problem when attempting to ssh, explicitly add your key: ssh-add ~/.ssh/id_rsa (or where ever your ssh key is). You might want to add this to the end of your shell .rc file

[christinach]

In my case I had an rsa key and a id_ed25519 key.
when I was trying to deploy with capistrano it was using the id_ed25519 and failing.
I was not able to set the desired order in the ~/.ssh/config file.
Adding both keys in the ssh agent didn't make any difference.
I removed the id_ed25519 and then I was able to deploy with cap successfully.