title: Doxygen: how to include a markdown page to document a group
body: <p>I have a rather complex project and I want to document it using doxygen.</p>
  
  <p>I have no problem documenting the code and I also managed to have a nice front-page using a custom <code>README.md</code> file coupled with "<code>USE_MDFILE_AS_MAINPAGE = README.md</code>" directive in Doxyfile.</p>
  
  <p>I defined several groups (<code>@defgroup</code>) which show up as "Modules" in my documentation.</p>
  
  <p>I would like to add a "main page" to each of the group giving general information, beside the customary function/variable/type documentation.</p>
  
  <p>I tried adding custom <code>MODULENAME.md</code> files coupled with matching <code>@includedoc MODULENAME.md</code> entries in group definition, it seem to work (I see several lines like: "<code>Generating docs for page md_mcu_noitr_coro_README...</code>"), but I cannot find if and where the page is linked (I expected to see it in the "Detailed Description" for the module, as it happens if I put some documentation inline where I put the "<code>@includedoc</code>" directive.</p>
  
  <p>a snippet of one of my modules is:</p>
  
  <pre><code>/**
   * @file coro.h
   * @brief definition of coroutine implementing functions.
   *
   * @date: Feb 8, 2018
   * @author: myself
   *
   * @defgroup coro "Coroutine implementation in plain 'C'."
   *
   * @includedoc mcu_noitr/coro/README.md
   * @{
   *
   */
  </code></pre>
  
  <p>What am I doing wrong?</p>
  
  <p><em>Note:</em> it is also a bit surprising I need to put the whole path from where my <code>Doxyfile</code> is, otherwise doxygen won't find it even if it's right beside the file containing the <code>@includedoc</code> command.</p>