Add config template plugin examples (#7)

plugins/kasane/.gitignore

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+vendor
+Kasanefile.lock

plugins/kasane/Kasanefile

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+layers:
+- ../../guestbook/guestbook-ui-deployment.yaml
+- patch.jsonnet
+- ../../guestbook/guestbook-ui-svc.yaml
+

plugins/kasane/README.md

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+# Kasane
+
+[Kasane](https://github.com/google/kasane) is a layering tool for Kubernetes which utilises Jsonnet for deep object modification and patching.
+
+Use following steps to try the application:
+
+* Follow instructions from [custom_tools.md](https://github.com/argoproj/argo-cd/blob/master/docs/custom_tools.md) to make sure `kasane` binary is available in `argocd-repo-server` pod.
+* Register `kasane` plugin `argocd-cm` ConfigMap:
+
+```yaml
+apiVersion: v1
+data:
+  configManagementPlugins: |
+    - name: kasane
+      init:
+        command: [kasane, update]
+      generate:
+        command: [kasane, show]
+```
+* Create application using `kasane` as a config management plugin name.
+
+```
+argocd app create kasane \
+    --config-management-plugin kasane \
+    --repo https://github.com/argoproj/argocd-example-apps \
+    --path plugins/kasane \
+    --dest-server https://kubernetes.default.svc \
+    --dest-namespace default
+```

plugins/kasane/patch.jsonnet

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+function (layers)
+
+[
+  layers[0] {
+    spec+: {
+      replicas: 2,
+    },
+  }
+]