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)
This commit is contained in:
jacob 2018-10-24 18:45:57 -05:00 committed by Jessica Parsons
parent 33e20ad751
commit 87768ee2c8

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@ -32,6 +32,10 @@ The Netlify CMS can be used in two different ways.
Netlify CMS has a [Gitter community](https://gitter.im/netlify/netlifycms) where members of the community hang out and share things about the project, as well as give and receive support.
# Contributing
New contributors are always welcome! Check out [CONTRIBUTING.md](https://github.com/netlify/netlify-cms/blob/master/CONTRIBUTING.md) to get involved.
# Change Log
This project adheres to [Semantic Versioning](http://semver.org/).