Add troubleshooting for Unifi Controller (home-assistant#4578)

Mike O'Driscoll · fabaff · commit c20709e3d09c · 2018-02-05T08:21:19.000+01:00
Added documentation regarding accurate time sync for presence detection
between home assistant and unifi controller.
diff --git a/source/_components/device_tracker.unifi.markdown b/source/_components/device_tracker.unifi.markdown
@@ -79,4 +79,11 @@ well.
 It is recommended that you run the Unifi controller in a dedicate
 virtual machine to avoid that situation.
 
+### {% linkable_title Troubleshooting and Time Synchronization %}
+
+Presence detection depends on accurate time configuration between Home Assistant and the Unifi controller.
+
+If Home Assistant and the Unifi controller are running on separate machines or VMs ensure that all clocks are syncronized.
+Failing to have syncronized clocks will lead to Home Assistant failing to mark a device as home.
+
 [Related Issue](https://github.com/home-assistant/home-assistant/issues/10507)
