Add a nice contributing file for anybody who wants to contribute to the project

[leordev]

@ErnieAtLYD do we have any CFM default standards to contributing policy?

[ErnieAtLYD]

@leordev Not a standard, but here are three from Code for America I think work. Take a look at them and adapt it to what works for you!

• https://github.com/codeforamerica/ohana-api/blob/master/CONTRIBUTING.md
• https://github.com/codeforamerica/streetmix/blob/master/CONTRIBUTING.md

[leordev]

Done here: https://github.com/Code-for-Miami/restaurant-inspections-api/blob/master/CONTRIBUTING.md