Add .DS_Store to .gitignore

Finder on OS X likes to leave lots of .DS_Store files everywhere, much
like a neighbourhood dog with an irresponsible owner. There's no way to
prevent these from re-appearing all the time; the best we can do is to
step over them while avoiding too close a glance ;)
diff --git a/.gitignore b/.gitignore
index edb74d6..700fbbf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,5 +16,8 @@
 
 # External downloads
 
+# Finder directory status files (OS X)
+.DS_Store
+
 
 #                    *** end of .gitignore ***