Added top-level readme with redirect to docs.

Author: jmattsson

README.md

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+# Powersensor integration for Home Assistant
+
+This repo contains the official [Powersensor](https://www.powersensor.com.au) integration for [Home Assistant](https://www.home-assistant.io).
+
+Please refer to the [documentation](https://dius.github.io/homeassistant-powersensor/) for details, including installation instructions.