Add API doc badge

Author: fhunleth

README.md

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 # Elixir Circuits - GPIO
 
-[![CircleCI](https://circleci.com/gh/elixir-circuits/circuits_gpio.svg?style=svg)](https://circleci.com/gh/elixir-circuits/circuits_gpio)
 [![Hex version](https://img.shields.io/hexpm/v/circuits_gpio.svg "Hex version")](https://hex.pm/packages/circuits_gpio)
+[![API docs](https://img.shields.io/hexpm/v/circuits_gpio.svg?label=hexdocs "API docs")](https://hexdocs.pm/circuits_gpio/Circuits.GPIO.html)
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 `Circuits.GPIO` lets you control or read from GPIOs on Nerves or other Linux-based
 devices.