Add a basic README.md

README.md

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+# Check Home Assistant State #
+
+A check plugin for [Nagios](https://www.nagios.org/)/[Icinga](https://icinga.com/)/etc. that queries [Home Assistant](https://www.home-assistant.io/) for the current state of entities.
+
+## Requirements ##
+
+- [Python](https://www.python.org/) >= 3.7
+- [requests](https://pypi.org/project/requests)
+- [nagiosplugin](https://pypi.org/project/nagiosplugin/)
+
+## Usage ##
+
+You will need a token for Home Assistant's REST API.
+[Instructions to aquire one can be found here](https://developers.home-assistant.io/docs/api/rest/).
+
+At a minimum, you need to specify the token, home assistant url, a device class to filter by, and warning/critical ranges.
+Example:
+```sh
+./check_home_assistant_state.py --token $HASS_TOKEN --url $HASS_URL --device-class battery --warning 20:100 --critical 10:100
+```
+See `--help` for more features/arguments.
+
+A CheckCommand config for Icinga2 can be generated with `--make-icinga-conf`.