Adding additional resources for CRDs and Controllers

Had several people asking about how to learn about
Custom Resource Definitions and Controllers with
Kubernetes.

Adding the best resources I've found on the above.

CONTRIBUTING.md

@@ -43,4 +43,20 @@ See the [Container Builder documentation](https://cloud.google.com/container-bui
 how to edit and expand the build process.
 
 ### Additional Resources
-- [How to write a good Git Commit message](https://chris.beams.io/posts/git-commit/) 
+
+#### Kubernetes
+
+- [Kubernetes Custom Resources](https://kubernetes.io/docs/concepts/api-extension/custom-resources/) - 
+  This is how we define our own resource names (`GameServer`, etc) within Kubernetes. 
+- [Joe Beda's TGIK Controller](https://github.com/jbeda/tgik-controller) -
+  [Joe Beda](https://twitter.com/jbeda) did a video series on writing controllers for Kubernetes.  
+  **This is the best resource for learning about controllers and Kubernetes.**
+- [Kubernetes Sample Controller](https://github.com/kubernetes/sample-controller) -
+  Example of a Custom Resources with a Kubernetes Controller.
+- [Kubernetes Code Generator](https://github.com/kubernetes/code-generator) -
+  The tooling that generated the Go libraries for the Custom Resource we define
+
+#### Coding and Development
+
+- [How to write a good Git Commit message](https://chris.beams.io/posts/git-commit/) -
+  Great way to make sure your Pull Requests get accepted.