Adds a Code of Conduct & Link in Contributing Docs

Coraline Ehmke gave a talk recently at Groupon's Geekfest about Diversity in Open Source Communities. She talked about how a Code of Conduct, when followed and applied, can help to foster openness and inclusivity in OSS. 

CoC's can act as signaling beacons to show that we care about contributions from everyone, and want to provide a safe space to learn, grow, and share knowledge. I know exercism was the first open source project I ever contributed to, and I would love for everyone to feel comfortable and able to contribute! :sparkling_heart: