Update packaging section in the README

README.md

@@ -16,7 +16,16 @@ My solutions to the [Advent of Code 2023](https://adventofcode.com/2023), writte
 
 ## Packaging
 
-The programs are packaged with Nix, making it easy for CI to run a single workflow over all days regardless of the compiler/language used.
+The programs are packaged with [Nix](https://nixos.org/), a functional package manager for Linux and macOS that focuses on reproducible builds. This makes it easy to build the programs, both locally and CI, without relying on system packages.
+
+To build one of the days, `cd` into the corresponding directory, `nix-build` the package and run it from the symlinked directory. For example, to build and execute day 4, run:
+
+```sh
+cd day04
+nix-build && result/bin/day04 resources/input.txt
+```
+
+Every day is packaged up to take exactly one command-line argument, the input file, and usually includes the demo input from the exercise too.
 
 ## Previous years