Since it took me a little while to get my head around how to use the gem buy I've found it really useful I thought that providing some getting started documentation could be a nice way of giving back.
I've added whatever I thought could be of interest for new comers but by all means please feel free to edit / adjust / complete. I also noted there is no mention to the LICENSE that guides is released under, it may be helpful to include that in the Readme or in a LICENSE file.
Provide some basic documentation
Since this is originally part of Rails, I suspect the license should be MIT. @wycats?