Add documentation for new Mullvad VPN integration (#14115)

Co-authored-by: Franck Nijhof <git@frenck.dev>

.textlintrc.json

@@ -172,6 +172,7 @@
         "MIME",
         "MJPEG",
         "MQTT",
+        "Mullvad",
         "MySensors",
         "NAS",
         "NETGEAR",

source/_integrations/mullvad.markdown

@@ -0,0 +1,19 @@
+---
+title: Mullvad VPN
+description: Instructions on how to integrate Mullvad VPN api data into Home Assistant.
+ha_category:
+  - Binary Sensor
+  - Network
+ha_iot_class: Cloud Polling
+ha_release: 2021.3
+ha_domain: mullvad
+ha_config_flow: true
+ha_codeowners:
+  - '@meichthys'
+---
+
+The Mullvad VPN integration pulls [Mullvad](https://mullvad.net/) VPN status information into Home Assistant.
+
+![](/images/screenshots/mullvad_vpn_sample_sensor_connected.png)
+
+{% include integrations/config_flow.md %}