README: document schema

Author: mweinelt

README.md

@@ -32,6 +32,18 @@ sensor:
         state_class: total_increasing
 ```
 
+## Options
+
+| Name                | Type   | Default | Description |
+|---------------------|--------|----------|-------------|
+| name                | string | required | Friendly name |
+| expr                | string | required | [PromQL](https://prometheus.io/docs/prometheus/latest/querying/basics/) expression |
+| unique_id           | string | optional | Unique ID |
+| unit_of_measurement | string | optional | Unit of the measurement |
+| device_class        | string | optional | [Device class](https://www.home-assistant.io/integrations/sensor/#device-class) |
+| state_class         | string | optional | [State class](https://developers.home-assistant.io/docs/core/entity/sensor/#available-state-classes) |
+
+
 ## Upstream integration
 
 I tried to get this component merged into home-assistant in 2020/12.