This document, like so much other great software out there, was created for free by volunteers. If you are at all knowledgeable about any aspect of gtkmm that does not already have documentation, please consider contributing to this document.
Ideally, we would like you to provide a merge request to the docs/tutorial/C/index-in.docbook file. This file is in the gtkmm-documentation module in GNOME git.
If you do decide to contribute, please post your contribution to the gtkmm mailing list at <email@example.com> or as an issue or merge request to GitLab. Also, be aware that the entirety of this document is free, and any addition you provide must also be free. That is, people must be able to use any portion of your examples in their programs, and copies of this document (including your contribution) may be distributed freely.