add blog link

Author: nerdralph

README.md

@@ -5,6 +5,9 @@ The Arduino Blink example sketch built for the Uno takes 924 bytes of flash, but
 ## Usage
 Download [a release](https://github.com/nerdralph/ArduinoShrink/releases), and extract the files into your Arduino/libraries folder.  Select ArduinoShrink from the Sketch, Include Library menu.  This will replace several Arduino functions with smaller and faster versions.  The Arduino pin number must be known at compile time if you are using this library.
 
+## Design and Implemenation
+I've written <a href="http://nerdralph.blogspot.com/2021/04/honey-i-shrunk-arduino-core.html">a blog post</a> discussing the implementation details of ArduinoShrink.
+
 ## Planned Improvements
 Update digitalWrite() to work when the pin is not known at compile time.