adds example using simple-app and downwardAPI

examples/simple-app-downward-api/app.yml

@@ -0,0 +1,43 @@
+apiVersion: kappctrl.k14s.io/v1alpha1
+kind: App
+metadata:
+  name: simple-app
+  namespace: default
+spec:
+  serviceAccountName: default-ns-sa
+  fetch:
+  - git:
+      url: https://github.com/k14s/k8s-simple-app-example
+      ref: origin/develop
+      subPath: config-step-2-template
+  template:
+  - ytt:
+      valuesFrom:
+      - downwardAPI:
+          items:
+            - name: ns
+              fieldPath: metadata.namespace
+            - name: k8sV
+              kubernetesVersion: {}
+            - name: kcV
+              kappControllerVersion: {}
+      inline:
+        paths:
+          filename.yaml: |
+            #@ load("@ytt:overlay", "overlay")
+            #@ load("@ytt:data", "data")
+            #@overlay/match by=overlay.subset({"kind": "Deployment"})
+            ---
+            spec:
+              template:
+                spec:
+                  containers:
+                  #@overlay/match by="name"
+                  - name: simple-app
+                    env:
+                    #@overlay/match by="name"
+                    - name: HELLO_MSG
+                      value: #@ "from {} namespace, deployed by kapp-controller v{}, hosted on kubernetes v{}".format(data.values.ns, data.values.kcV, data.values.k8sV)
+  #! sample output: <h1>Hello from default namespace, deployed by kapp-controller v0.40.0+develop, hosted on kubernetes v1.23.3!</h1>
+  deploy:
+  - kapp: {}