Bring back CODEOWNERS (with me).

Not enough people are active right now for me to get quick code reviews, so
I'm going back to the original model where I review everything and my commits
don't have to get reviews. It's not ideal but I think it's the best choice
for the current situation.

CODEOWNERS

@@ -0,0 +1,6 @@
+# Define who is responsible for reviewing what code.
+#
+# For now, n-rook should review all changes made to the repository. The plan
+# is to add more people to this list if and when they get comfortable
+# writing code and get used to doing code reviews.
+* @n-rook