Add README.md

Author: Mr-Ker

README.md

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+# Advent of Code 2024
+
+The provided solutions for the 2024 edition of Advent of Code are implemented in
+Rust.
+
+## Run for a specific day
+
+`cargo run -- --day N --input-folder <some_folder>`
+
+The program will look for a text file named `dayXX.txt`, where `XX` is the day
+number led by `0` for days `< 9`, within the provided `<input_folder>` and use
+it as input file.
+
+## Run the tests
+
+`cargo test`