Update frontend_creating_custom_ui.markdown (home-assistant#3275)

source/developers/frontend_creating_custom_ui.markdown

@@ -13,22 +13,27 @@ If you would like to use your own [State card](/developers/frontend_add_card/) w
 
 Put the element source file and its dependencies in `www/custom_ui/` directory under your Home Assistant [configuration](/docs/configuration/) directory.
 
-For example if creating a state card for the `light` domain named `my_custom_light_card` put `state-card-my_custom_light_card.html` in `www/custom_ui/`.
+For example if creating a state card for the `light` domain named `my_custom_light_card` put `my_custom_light_card.html` in `www/custom_ui/`.
 
-That file should implement `<state-card-my_custom_light_card>` tag with Polymer.
+That file should implement `<my_custom_light_card>` tag with Polymer.
 
-In `state-card-my_custom_light_card.html` you should use `<link rel="import">` to import all the dependencies **not** used by Home Assistant's UI.
+In `my_custom_light_card.html` you should use `<link rel="import">` to import all the dependencies **not** used by Home Assistant's UI.
 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.
 
-In the `customize:` section of the `configuration.yaml` file put `custom_ui_state_card: <element-name>`.
+1) In the `customize:` section of the `configuration.yaml` file put `custom_ui_state_card: <element-name>`.
+2) In the `frontend` section use `extra_html_url` to specify the URL to load.
 
 For example:
 ```yaml
 homeassistant:
   customize:
-    - entity_id: light
+    light.bedroom:
       custom_ui_state_card: my_custom_light_card
+
+frontend:
+  extra_html_url:
+    - /local/custom_ui/my_custom_light_card.html
 ```
 
 For more possibilities, see the [Custom UI section](/cookbook/#user-interface) on our Examples page.