Please document how to open an interactive console with a deployed contract

[emchristiansen]

Please document how to open an interactive console with a deployed contract.

I'd like to play with a contract interactively (like Truffle advertises here), but I've spent 30 minutes trying different commands and I keep running into issues.
(But, I am a JavaScript ecosystem noob.)
Please show how to do this in the README, assuming it can be done.

Thanks!