Adds README and first Minitest

@AdamSmith910 - You're totally on the right track here! I think it might help you to move forward if you can add a little bit of testing - so this pull request adds a very basic README with the testing instructions and your first test on Game.rb

I'm guessing setting up testing was a little difficult because you have a lot of 'puts' commands and because you have your classes namespaced under Mastermind (good work on that!) ---- But hopefully this clears it up a little bit!

Feel free to hit me up on how to merge this PR if you want too!

Good code!