Document Nix solution

2025/day01/README.md

@@ -0,0 +1,17 @@
+# Day 01: Nix
+
+Nix is a functional language made for the Nix package manager. As 
+
+To run the solution program, start the `nix` repl with the solution program and call the `solve`
+function with the path to the input file.
+
+```console
+$ nix repl --option max-call-depth 10000 --file solve.nix
+nix-repl> solve ./sample.txt
+```
+
+Some observations:
+
+- The `max-call-depth` needs to be bumped to at least 10k for the main input files, otherwise you
+  hit the limit in the recursion.
+- The standard library is lacking several basics but most of those you can build yourself