Added HTML document link.

Author: suikan4github

README.md

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 # Raspberry Pi Pico Driver class collection
 A Collection of the Raspberry Pi [Pico](https://www.raspberrypi.com/products/raspberry-pi-pico/)/[Pico2](https://www.raspberrypi.com/products/raspberry-pi-pico-2/) classes. Also, the [SDK API](https://www.raspberrypi.com/documentation/pico-sdk/hardware.html#group_sm_config_1gaed7a6e7dc4f1979c7c62e4773df8c79b) wrapper class is provided. 
 
+[HTML documentation](https://suikan4github.github.io/rpp_driver/) is available. 
+
 # Details
 This class collection ( or class library ) encapsule the certain data inside class. And it also allows you to use the dependency-injection inside your project. So, you can test your code with [Google Test](https://github.com/google/googletest) before testing on the target hardware.