Create a CODEOWNERS file (#19)

This makes it easier for people to see who the active maintainers of
this package are.

If someone stops being an active maintainer, they can remove themselves
from the `CODEOWNERS` list, and then when they come back as an active
maintainer, it's quite easy for them to add themselves back to the
`CODEOWNERS` file.

Author: DilumAluthge

CODEOWNERS

@@ -0,0 +1,3 @@
+# Usernames in CODEOWNERS must start with @
+
+* @DrChainsaw @bjarthur