Missing "eval `ssh-agent -s`" step in README.md

[five-vee]

It seems that Mac OS X users already have ssh-agent -s running in the background, but not for Ubuntu, Windows Cygwin, or Windows 10 Ubuntu Bash. It should be explicitly stated that Linux users should make sure it is running before trying to set-up passwordless ssh through peg.

This has been a major pain point for us (permission denied (public key) errors abound).

[matthewlamdotjs]

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