github: add a CODEOWNERS file with only the maintainers in

As discussed several times before, we want to restrict permission to
approve PRs to the maintainers only. This patch adds a GitHub
CODEOWNERS file for that purpose. Once this has been merged, I'm going
to update the rulesets to make PRs requires approval from a
maintainer.

.github/CODEOWNERS

@@ -0,0 +1,3 @@
+# The maintainers own all files.
+# See GOVERNANCE.md for the list of current maintainers.
+* @jj-vcs/maintainers