README: Include more verbose instructions for developers.

Hey @allantokuda , thanks for the awesome demo today, I think this tool is going to really help the class out during the semester.  I've updated the README to provide a more detailed setup/installation procedure for those who are not familiar with the ruby stack.
I probably wasn't clear. You'll see `.ruby-version` is already included in the project when you check it out, so this step is not necessary.