Add ColPrac to README (#156)

We're already following this, we just hadn't added the badge.

Author: willtebbutt

README.md

@@ -4,6 +4,7 @@
 [![Coverage Status](https://coveralls.io/repos/github/JuliaGaussianProcesses/KernelFunctions.jl/badge.svg?branch=master)](https://coveralls.io/github/JuliaGaussianProcesses/KernelFunctions.jl?branch=master)
 [![Documentation (stable)](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliagaussianprocesses.github.io/KernelFunctions.jl)
 [![Documentation (latest)](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliagaussianprocesses.github.io/KernelFunctions.jl/dev)
+[![ColPrac: Contributor's Guide on Collaborative Practices for Community Packages](https://img.shields.io/badge/ColPrac-Contributor's%20Guide-blueviolet)](https://github.com/SciML/ColPrac)
 
 ## Kernel functions for machine learning