Update README.md

Author: theznerd

2021/README.md

@@ -18,3 +18,4 @@ Anthony Fontanez|https://github.com/ajf8729/AoC-2021
 Chris Dent|https://github.com/indented-automation/AoC/tree/master/2021/01
 Adam Cook|https://github.com/codaamok/AoC/tree/main/2021
 Mamue|https://github.com/Maamue/AoC/tree/main/2021
+Nathan Ziehnert|https://github.com/theznerd/AdventOfCode/tree/main/2021