Insist that the cisagov devs are the owners of the .github directory

This additional clause must remain at the _end_ of the CODEOWNERS file
so that it cannot be overridden by a later clause.

We want to make it so that all the .github files including CODEOWNERS
are protected so only code owners (the dev team) can approve
modifications to them.

This will prevent configuration changes from breaking Actions and
other management-type functions that the files in this directory
control. By setting the .github files/folder to require code owner
approval for changes, workflow and management changes will require dev
team review and checking.

Resolves #56.

.github/CODEOWNERS

@@ -1,7 +1,10 @@
 # Each line is a file pattern followed by one or more owners.
 
-# These owners will be the default owners for everything in
-# the repo. Unless a later match takes precedence,
-# these owners will be requested for review when someone
-# opens a pull request.
+# These owners will be the default owners for everything in the
+# repo. Unless a later match takes precedence, these owners will be
+# requested for review when someone opens a pull request.
 * @dav3r @felddy @hillaryj @jsf9k @mcdonnnj
+
+# These folks own any files in the /.github directory at the root of
+# the repository and any of its subdirectories.
+/.github/ @dav3r @felddy @hillaryj @jsf9k @mcdonnnj