Add CODEOWNERS

This is so people in the @RDFLib/core team can see pull requests in
<https://github.com/pulls/review-requested>.

I'm not entirely sure if this will work because @RDFLib/core is private,
but worth a try I think.

Author: aucampia

CODEOWNERS

@@ -0,0 +1 @@
+*       @RDFLib/core