README: Replace ForwardAgent with ProxyCommand
README: Replace ForwardAgent with ProxyCommand

Using `ProxyCommand` is preferable to `ForwardAgent` because it doesn't
expose your local agent to other users on the remote machine.

This is a reasonable blog post, for further reading on about the issue:

- https://heipei.github.io/2015/02/26/SSH-Agent-Forwarding-considered-harmful/

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