Add initial docs for panel_custom (#750)

source/_components/panel_custom.markdown

@@ -0,0 +1,40 @@
+---
+layout: page
+title: "Panel Custom"
+description: "Instructions how to add customied panels to the frontend of Home Assistant."
+date: 2015-08-08 11:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Frontend
+ha_release: 0.26
+---
+
+
+The `panel_custom` support allows you to add additional panels to your Home Assistant frontend. The panels are listed in the sidebar if wished and can be highly customized.
+
+To enable customized panels in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+panel_custom:
+  - name: todomvc
+    sidebar_title: TodoMVC
+    sidebar_icon: mdi:work
+    url_path: my-todomvc
+    webcomponent_path: /home/hass/hello.html
+    config:
+      hello: world
+```
+
+Configuration variables:
+
+- **name** (*Optional*): Name of the panel.
+- **sidebar_title** (*Optional*): Friendly title for the panel in the sidebar. Omitting it means no sidebar entry (but still accessible through the URL).
+- **sidebar_icon** (*Optional*): Icon for entry. Pick an icon that you can find on [materialdesignicons.com](https://materialdesignicons.com/) to use for your input and prefix the name with `mdi:`. For example `mdi:car`, `mdi:ambulance`, or  `mdi:motorbike`.
+- **url_path** (*Optional*): The URL your panel will be available on. If omitted will default to the panel name.
+- **webcomponent_path** (*Optional*): The  path to your component. If omitted will default to <config dir>/panels/<component name>.html
+- **config** (*Optional*): Configuration to be passed into your web component when being instantiated.
+