Update readme

README.md

@@ -1,2 +1,4 @@
-# adventofcode2018
-Working out solutions for Advent of Code 2018
+# adventofcode
+Working out solutions for Advent of Code.
+
+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.)