Add documentation for Discogs sensor (#4138)

* Add documentation for Discogs sensor

* Update config style

source/_components/sensor.discogs.markdown

@@ -0,0 +1,39 @@
+---
+layout: page
+title: Discogs Sensor
+description: "Instructions on how to set up Discogs sensors within Home Assistant."
+date: 2017-12-04 22:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: https://www.magneticmag.com/.image/t_share/MTQ5OTE1NzU4NTczMzk3OTYw/discogs-vinyl-record-mark.png
+ha_category: Sensor
+ha_release: 0.61
+ha_iot_class: "Cloud Polling"
+---
+
+The `discogs` platform allows you to see the current amount of records in your [Discogs](https://discogs.com) collection.
+
+First, you'll need to get a personal access token from your Discogs account.
+You can generate a token from your profile's [Developer settings](https://www.discogs.com/settings/developers).
+
+To enable this sensor, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: discogs
+    token: YOUR_TOKEN
+```
+
+{% configuration %}
+token:
+  description: The Discogs API token to use as identification to get your collection.
+  required: true
+  type: string
+name:
+  description: Name to use in the frontend.
+  required: false
+  type: string
+{% endconfiguration %}