docs: make it clear that kustomize is required for unit tests (argoproj#1484)

* docs: make it clear that kustomize is required for unit tests

Signed-off-by: Kostis Kapelonis <[email protected]>

* docs: make it clear that kustomize is required for unit tests

Signed-off-by: Kostis Kapelonis <[email protected]>

Author: kostis-codefresh

docs/CONTRIBUTING.md

@@ -11,6 +11,9 @@ Install:
 * [kustomize](https://github.com/kubernetes-sigs/kustomize/releases)
 * [minikube](https://kubernetes.io/docs/setup/minikube/) or Docker for Desktop
 
+Kustomize is required for unit tests (`make test` is using it), so you [must install it](https://kubectl.docs.kubernetes.io/installation/kustomize/)
+locally if you wish to make code contributions to Argo Rollouts.
+
 Argo Rollout additionally uses the following tools
 
 * `golangci-lint` to lint the project.