Add README

Author: oxan

README.md

@@ -1,11 +1,54 @@
 # HAPT
 
-Home Assistant Presence Tracker (HAPT) is an event-driven device presence tracker for [Home Assistant](https://www.home-assistant.io/) on an OpenWRT router or access point.
+Home Assistant Presence Tracker (HAPT) is an event-driven device presence tracker for [Home Assistant][homeassistant] on
+an OpenWRT router or access point.
 
 ## Description
 
-Coming soon...
+HAPT listens on association and disassociation events to wireless networks, using the hostapd control interface. It
+keeps track of which device is connected to which networks. When a device connects to its first network, or disconnects
+from its last network, a service call to Home Assistant is performed to mark the device as home or away. By tracking
+active device connections, HAPT ensures that a device switching between different networks (e.g. the 2.4 GHz and 5 GHz
+bands) is not marked as away.
+
+## Usage
+
+### Installation
+Download a package from the [releases][releases] page, and install it either by uploading it in LuCi (System > Software)
+or running `opkg install <file>` from a shell.
+
+### Configuration
+Once the package is installed, you must update the configuration in `/etc/config/hapt`. At minimum the `host` option
+should be set to the address of your Home Assistant installation, and the `token` option to a Home Assistant
+[long-lived access token][token] (these can be generated in the Home Assistant web interface).
+
+The `consider_home_timeout` and `consider_home_disconnect` settings can be used to configure for how long (in seconds)
+after the first association and last disassociation event the device should be considered home. Since Home Assistant
+does not support marking a device as away (on disconnects), this is implemented by marking the device as home for a
+negligible amount of time, after which Home Assistant will mark the device as away. The default values should be fine
+here.
+
+With the `wifi_interfaces` option, it is possible to specify the wireless interfaces that must be monitored. This can be
+used (for example) to ignore devices on a guest network.
+
+### Running
+After modifying the configuration, you must restart the service by `service hapt restart`. This can also be done from
+the LuCi interface (System > Startup). HAPT prints log messages to the system log, so that you can verify it is working
+as expected.
+
+It is possible to synchronize just the currently connected devices with Home Assistant by running `hapt` from the
+command line. This can be especially useful to debug the connection with Home Assistant, as this will also print any
+errors that occur.
+
+## Development
+You can build a custom package by running the `makepkg.sh` script, which will run the package build in a Docker
+container and place the compiled package in the `build/bin` directory.
 
 ## Acknowledgments
 
-This project has been inspired by the [openwrt_hass_devicetracker](https://github.com/mueslo/openwrt_hass_devicetracker) package.
+This project has been inspired by the [openwrt_hass_devicetracker][hasstracker] package.
+
+[homeassistant]: https://www.home-assistant.io/
+[hasstracker]: https://github.com/mueslo/openwrt_hass_devicetracker
+[releases]: https://github.com/oxan/hapt/releases
+[token]: https://www.home-assistant.io/docs/authentication/