Add docs for the syncthing integration (#14160)

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

CODEOWNERS

@@ -460,6 +460,7 @@ source/_integrations/switchbot.markdown @danielhiversen
 source/_integrations/switcher_kis.markdown @tomerfi
 source/_integrations/switchmate.markdown @danielhiversen
 source/_integrations/syncthru.markdown @nielstron
+source/_integrations/syncthing.markdown @zhulik
 source/_integrations/synology_dsm.markdown @hacf-fr @Quentame @mib1185
 source/_integrations/synology_srm.markdown @aerialls
 source/_integrations/syslog.markdown @fabaff

source/_integrations/syncthing.markdown

@@ -0,0 +1,35 @@
+---
+title: Syncthing
+description: Instructions on how to integrate Syncthing within Home Assistant.
+ha_category:
+  - Downloading
+  - Sensor
+ha_release: 2021.06
+ha_iot_class: Local Polling
+ha_quality_scale: silver
+ha_config_flow: true
+ha_codeowners:
+  - '@zhulik'
+ha_domain: syncthing
+---
+
+[Syncthing](https://syncthing.net/) is a continuous file synchronization program. It synchronizes files between two or more computers 
+in real-time, safely protected from prying eyes.
+
+The Syncthing integration allows you to monitor states of your synced folders from within Home Assistant and set up automations based on the information.
+
+## Prerequisites
+
+To set up the monitoring you need to get an **API key**. Open the Syncthing web 
+interface(e.g., `http://127.0.0.1:8384`) in the browser and go to **Actions** -> **Settings**. You will find
+the key on the right of the settings dialog.
+
+{% include integrations/config_flow.md %}
+  
+## Integration Entities
+
+The Syncthing integration adds one sensor per syncing folder:
+
+![Syncthing Sensors](/images/integrations/syncthing/sensors.png)
+
+![Syncthing Sensors](/images/integrations/syncthing/sensor.png)