update readme progress

Author: UnderscoreTud

README.md

@@ -17,6 +17,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 6](https://adventofcode.com/2024/day/6) | ⭐ | ⭐ |
 | [Day 7](https://adventofcode.com/2024/day/7) | ⭐ | ⭐ |
 | [Day 8](https://adventofcode.com/2024/day/8) | ⭐ | ⭐ |
+| [Day 9](https://adventofcode.com/2024/day/9) | ⭐ | ⭐ |
 <!--- advent_readme_stars table --->
 
 <!--- benchmarking table --->