Add README

Author: Nick Veys

README.md

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+# homebridge-dht-rpi
+
+DHT11/22 plugin for [Homebridge][homebridge] running on a Raspberry Pi GPIO pin.  Provides temperature and relative humidity.
+
+## Installation
+
+1. Install this plugin using: npm install -g homebridge-dht-rpi
+1. Edit ``config.json`` and add the sensor(s).
+1. Run Homebridge
+
+### Config.json Example
+
+...
+
+#### Optional Parameters
+
+* `type` type of device, either `"DHT11"` or `"DHT22"`
+* `pin` GPIO pin number, default `4`
+
+[homebridge]: https://github.com/nfarina/homebridge