Just in case you didn't find it yyet, there is a FAQ at https://www.geany.org/documentation/faq/, as @elextr said if there is something missing, feel free to create a pull request at https://github.com/geany/www.geany.org. The source of the FAQ page is at https://github.com/geany/www.geany.org/blob/master/page_content/documentatio....
For further customization you can also have a look at the Wiki, e.g. https://wiki.geany.org/howtos/start.
Admittedly, the information are a bit spread at the moment. Maybe we can improve this in the future.
Linking the FAQ in the project README seems like a good idea to me.