Add Lanternfish lesson

Author: fwcd

README.md

@@ -47,6 +47,8 @@ Every day is packaged up to take exactly one command-line argument, the input fi
 ## Lessons Learned
 
 - Dijkstra, Dijkstra, Dijkstra (day [16](day16), [18](day18), [20](day20))
+- The [Lanternfish](https://adventofcode.com/2021/day/6)-style counting trick seems to be useful (day [11](day11), [21](day21))
+  - See e.g. also [this Reddit post](https://www.reddit.com/r/adventofcode/comments/1hbn15t/2024_day_11_is_this_a/)
 
 ## Previous Years
 

README.md.gyb

@@ -54,6 +54,8 @@ Every day is packaged up to take exactly one command-line argument, the input fi
 ## Lessons Learned
 
 - Dijkstra, Dijkstra, Dijkstra (${day_links([16, 18, 20])})
+- The [Lanternfish](https://adventofcode.com/2021/day/6)-style counting trick seems to be useful (${day_links([11, 21])})
+  - See e.g. also [this Reddit post](https://www.reddit.com/r/adventofcode/comments/1hbn15t/2024_day_11_is_this_a/)
 
 ## Previous Years