Add 2021 Description

README.md

@@ -5,6 +5,14 @@
 Advancing knowledge of Kotlin, and focusing on using Sequences where possible
 for efficiently generating answers.
 
+## 2021
+
+Originally, I started AoC 2021 in Rust however I had set out to do it purely with
+immutability. This ended up being a bad idea, especially with a limited knowledge
+of the language at the time. Here on a number of years later, it's been completed
+in Kotlin. There was also a larger focus on being able to generalize code for
+re-use for use in future solutions.
+
 ## 2022
 
 I originally started AoC 2022 in Clojure, and ported it