Add dependencies retrieval for helm chart development preparation

Without it the validation will fail with a message
that might not be clear for new helm chart developers.

Author: sgaist

CONTRIBUTING.md

@@ -194,6 +194,13 @@ continue.
    (cd helm-chart && chartpress)
    ```
 
+1. Get the chart dependencies (for example JupyterHub)
+
+   ```bash
+   cd helm-chart/binderhub
+   helm dependency update
+   ```
+
 1. Validate, and then install the Helm chart defined in helm-chart/binderhub.
 
    This validation step is not making any modification to your Kubernetes
@@ -209,6 +216,8 @@ continue.
 
    If the validation succeeds, go ahead and upgrade/install the Helm chart.
 
+   Note that this will do the installation in the current namespace.
+
    ```bash
    helm upgrade --install binderhub-test helm-chart/binderhub \
       --values testing/k8s-binder-k8s-hub/binderhub-chart-config.yaml \