docs: Add CONTRIBUTING.md link to README.md (#1822)
docs: Add CONTRIBUTING.md link to README.md (#1822)

I felt like this document should be referenced in the README so that new users don't have to go looking for it.

**Summary**

I had to look around for this file, some people might not!

**A picture of a cute animal (not mandatory but encouraged)**
Me and my dogs:
![27575817_182959932310538_3964599695786901504_n](https://user-images.githubusercontent.com/11466782/47262251-f5e7bf00-d4a8-11e8-8c37-55e8215e23f1.jpg)



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