Spell out review-within-two-weeks rule for Code Owners

Pull requests that do not receive timely feedback block progress and cause
unnecessary overhead as repeated rebasing is necessary and increasingly costly.

Author: tautschnig

CODEOWNERS

@@ -1,3 +1,6 @@
+# Code owners are expected to complete reviews with feedback to the PR author or
+# approval within two weeks.
+#
 # These owners will be the default owners for everything in the repo.
 *       @kroening @tautschnig @peterschrammel @smowton @chrisr-diffblue