custom-ui state-card upgrade example (#3698)

* custom-ui state-card upgrade example

* wording

* Add some line breaks

source/developers/frontend_creating_custom_ui.markdown

@@ -9,6 +9,7 @@ sharing: true
 footer: true
 ha_release: 0.38
 ---
+
 If you would like to use your own [State card](/developers/frontend_add_card/) without merging your code into [home-assistant-polymer](https://github.com/home-assistant/home-assistant-polymer/) you can create your own implementation.
 
 Put the element source file and its dependencies in `www/custom_ui/` directory under your Home Assistant [configuration](/docs/configuration/) directory.
@@ -21,10 +22,13 @@ In `state-card-my-custom-light.html` you should use `<link rel="import">` to imp
 Do not import any dependencies used by the Home Assistant UI.
 Importing those will work in `development: 1` mode, but will fail in production mode.
 
-1) In the `customize:` section of the `configuration.yaml` file put `custom_ui_state_card: state-card-my-custom-light`.
-2) In the `frontend` section use `extra_html_url` to specify the URL to load.
+1. In the `customize:` section of the `configuration.yaml` file put `custom_ui_state_card: state-card-my-custom-light`.
+2. In the `frontend` section use `extra_html_url` to specify the URL to load.
+
+Example:
+
+`configuration.yaml`:
 
-For example:
 ```yaml
 homeassistant:
   customize:
@@ -36,4 +40,42 @@ frontend:
     - /local/custom_ui/state-card-my-custom-light.html
 ```
 
+`www\custom_ui\state-card-my-custom-light.html`:
+
+```javascript
+<dom-module id='state-card-my-custom-light'>
+  <template>
+    <style>
+
+    </style>
+    <textarea>[[_toStr(StateObj)]]</textarea>
+  </template>
+</dom-module>
+
+<script>
+class StateCardMyCustomLight extends Polymer.Element {
+  static get is() { return 'state-card-my-custom-light'; }
+  
+  static get properties() {
+    return {
+      // Home Assistant object
+      hass: Object,
+      // inDialog is true if shown as more-info-card
+      inDialog: {
+        type: Boolean,
+        value: false,
+      },
+      // includes state, config and more information of the entity
+      stateObj: Object,
+    };
+  }
+
+  _toStr(obj) {
+    return JSON.stringify(obj);
+  }
+}
+customElements.define(StateCardMyCustomLight.is, StateCardMyCustomLight);
+</script>
+```
+
 For more possibilities, see the [Custom UI section](/cookbook/#user-interface) on our Examples page.