Add documentation for the LG soundbar component (#6919)

* Add documentation for the LG soundbar component

* ✏️ Adds ha_iot_class

source/_components/media_player.lg_soundbar.markdown

@@ -0,0 +1,26 @@
+---
+layout: page
+title: "LG Soundbars"
+description: "Instructions on how to integrate LG Soundbars into Home Assistant."
+date: 2018-10-17 19:55:18
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: lg.png
+ha_category: Media Player
+ha_iot_class: "Local Polling"
+ha_release: 0.81
+---
+
+The `lg_soundbar` platform allows you to control [LG Soundbars](https://www.lg.com/us/sound-bars) from Home Assistant.
+
+Supported devices:
+
+- The SK range
+
+Compatible devices will be automatically added if the [`discovery`](/components/discovery/) component is enabled.
+
+### {% linkable_title Supported operations %}
+
+- These devices support mute, volume control, source selection and mode selection.