How to include custom files in Doxygen

Just for completeness: there are 3 possible extensions which doxygen treats as additional documentation files: .dox, .txt, and .doc.

Files which such extension are hidden from the file index. Inside the file you need to put one or more C/C++ style comment blocks.

I having been searching quite a lot before I found the answer, so I thought it would be nice to share!

According to this Doxygen gotchas article I finally found that: you need to add a file with the dox extension. Its content should contain C-style comment blocks:

/*!
  \page My test page
  contents
  ...
  more contents
*/

Make sure your custom files are placed in a directory which is included in INPUT setting or in the current directory if INPUT is left empty, so these files can be found.