Add CODEOWNERS to auto-request andreogle as reviewer

Dependabot respects CODEOWNERS, so every Dependabot PR now requests
review from the repo owner. The catch-all covers root deps, book/ deps,
and GitHub Actions.

Author: andreogle

.github/CODEOWNERS

@@ -0,0 +1,7 @@
+# Code owners are automatically requested for review on pull requests.
+# Dependabot respects CODEOWNERS, so every Dependabot PR (dependency
+# manifests, lockfiles, and GitHub Actions) requests review from the owner.
+#
+# Syntax: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+
+* @andreogle