Setup instructions for Mac

[brynte]

As a user that almost never used the terminal before I'm quite proud to get it working. However it took som googling to find solutions for a couple of things in the instructions. I'll just drop what I found here for others to see, or if you'd like to update the instructions.

before 'rvm install 2.3.1' add 'source /Users/[user]/.rvm/scripts/rvm'

At the bundle install, change to: 'bundle install --without production'

I'm on OSx 10.11.6 and this works on all three machines I'm using.

Thanks for an awesome tool!

[dphuang2]

I will remember to add that to the README. Thanks for your input 👍