update readme progress

Author: UnderscoreTud

README.md

@@ -23,6 +23,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 12](https://adventofcode.com/2024/day/12) | ⭐ | ⭐ |
 | [Day 13](https://adventofcode.com/2024/day/13) | ⭐ | ⭐ |
 | [Day 14](https://adventofcode.com/2024/day/14) | ⭐ | ⭐ |
+| [Day 15](https://adventofcode.com/2024/day/15) | ⭐ | ⭐ |
 <!--- advent_readme_stars table --->
 
 <!--- benchmarking table --->