Add CODEOWNERS (heroku#73)

For parity with our other getting started guides. Docs: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners GUS-W-14825333.

Author: edmorley

.github/CODEOWNERS

@@ -0,0 +1,9 @@
+# Note: Delete this file if you are copying the code in this repository into your own project.
+
+# Default to requesting pull request reviews from the Heroku Languages team.
+* @heroku/languages
+
+# However, request review from the Heroku language owner for files that are updated
+# by Dependabot, to reduce team review request noise.
+composer.json @dzuelke
+composer.lock @dzuelke