Warning about not using latest spago.

[martyall]

I just wanted to report a user experience problem. I was introducing my friend to purescript through this book and unfortunately because of the new purescript-based spago which uses yaml files, you have to be careful when installing spago globally and using this book. I would recommend updating some kind of warning that this requires a certain spago version dependency. Alternatively it would be nice to just include the one-liner to start a nix shell with the necessary node + purescript + spago dependencies you are assuming, because in this particular case it would have saved him a lot of headache.