Add link to AOC

README.md

@@ -1,4 +1,4 @@
 # adventofcode
-Working out solutions for Advent of Code.
+Working out solutions for [Advent of Code](https://adventofcode.com/).
 
 If you encounter a file with a .txt or .in extension, those are *my* inputs; they won't work for you! (This is not possessiveness, it's just the way the site is programmed.)