Clarify Marco's commitment (#443)

Author: marcoieni

CONTRIBUTING.md

@@ -115,3 +115,13 @@ history and make it easier for us to see changes between reviews.
 
 Make sure to `Allow edits of maintainers` (under the text box) in the PR so
 people can actually collaborate on things or fix smaller issues themselves.
+
+## Maintainers
+
+This repository is maintained by the following people:
+
+- [simonsan](https://github.com/simonsan).
+- [marcoieni](https://github.com/marcoieni). Marco only works on the book CI. He
+  doesn't review the book content and he doesn't receive notifications on new
+  issues and pull requests. If you want him to have a look at a CI issue, ping
+  him directly.