Adding details of merging trackers (home-assistant#6000)

* Adding details of merging trackers

Since the merging of trackers works for more than just OwnTracks (at least GPS Logger, probably more), adding the details here at the top level.

* Minor changes

Author: DubhAd

source/_components/device_tracker.markdown

@@ -29,7 +29,6 @@ device_tracker:
     new_device_defaults:
       track_new_devices: True
       hide_if_away: False
-
 ```
 
 The following optional parameters can be used with any platform. However device tracker will only look for global settings under the configuration of the first configured platform:
@@ -89,6 +88,24 @@ devicename:
 | `hide_if_away` | False                         | If `yes`/`on`/`true` then the device will be hidden if it is not at home.                                |
 | `consider_home` | [uses platform setting]      | Seconds to wait till marking someone as not home after not being seen. Allows you to override the global `consider_home` setting from the platform configuration on a per device level.                                 |
 
+## {% linkable_title Using GPS device trackers with local network device trackers %}
+
+GPS based device trackers (like [OwnTracks](/components/device_tracker.owntracks/), [GPSLogger](/components/device_tracker.gpslogger) and others) can also be used with local network device trackers, such as [Nmap](/components/device_tracker.nmap_tracker/) or [Netgear](/components/device_tracker.netgear/). To do this, fill in the `mac` field to the entry in `known_devices.yaml` with the MAC address of the device you want to track. This way the state of the device will be determined by *the source that reported last*. The naming convention for known device list is `<username>_<device-id>` and could be set in the app configuration.
+
+An example showing the inclusion of the `mac` field for multiple platform tracking. The `mac` field was added to the GPS based device tracker entry and will enable tracking by all platforms that track via the `mac` address.
+
+```yaml
+USERNAME_DEVICE_ID:
+  name: Friendly Name
+  mac: EA:AA:55:E7:C6:94
+  picture: https://www.home-assistant.io/images/favicon-192x192.png
+  gravatar: [email protected]
+  track: yes
+  hide_if_away: no
+```
+
+If you want to track whether either your GPS based tracker or your local network tracker, identify you as being at home, use [a group](/components/group/) instead.
+
 ## {% linkable_title Device states %}
 
 The state of your tracked device will be `'home'` if it is in the [home zone](/components/zone#home-zone), detected by your network or Bluetooth based presence detection. If you're using a presence detection method that includes coordinates then when it's in a zone the state will be the name of the zone (in lower case). When a device isn't at home and isn't in any zone, the state will be `'not_home'`.