Added README

Signed-off-by: Louis Vallat <louis@louis-vallat.xyz>

README.md

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+# Advent Of Code 2021
+
+> Find the sleigh key to save Santa, using your smol submarine!
+
+## What
+
+This repository hosts the code that I produced to solve the different puzzles
+provided for the Advent of Code 2021, that you can find [here](https://adventofcode.com/2021).
+
+## How
+
+In order to solve the different puzzles, I used the **Rust** programming language,
+as I wanted a way to experiment and practice this new language.
+
+There are no dependencies and no external crate.
+
+To execute it, just open the specific day you want to run, and use `cargo run --release -- ./input`.
+
+Replacing the `input` file **should** work without any code modification.
+
+## Why
+
+Because puzzles are fun! And Rust is fun too!
+
+## Yikes
+
+The code produced may not be optimal nor the fastest. And that's normal, I tried
+to spend less than a day (and to do it in my spare time after school).
+
+The code is also very ugly for a reason: usually I use single letters for the
+variable names. Usually it's fine. For me. For other people re reading me, it's
+less fun. So I forced myself to use mainly single characters as variable names,
+just to make me want to stop doing it.
+