Add Master codeowners

In conjunction with branch protection rules, this will require
approval from the Org owners (myself or @nvllsvm) before a merge
can be accepted.

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+# Jellyfin CODEOWNERS file (`master` branch)
+
+# Require org owner approvals for all files, when merging to master
+*   @joshuaboniface @nvllsvm