Update readme with dependency info

README.md

@@ -3,4 +3,6 @@
 Hooks for the pacman package manager that keep the package cache clean.
 
 - When a package is upgraded: All except its two newest package files (i.e. the current and the one before) are purged.
-- When a package is removed: All of its package files are purged.
+- When a package is removed: All of its package files are purged.
+
+The package `pacman-contrib` is required for `paccache`, which is used to actually clean the package cache.