message: add a link to custom compilation options
diff: @@ -61,3 +61,8 @@ PCM API documentation
  --------------------------------------------------------------------------------
  PCM API documentation is embedded in the source code and can be generated into html format from source using Doxygen (www.doxygen.org).
  +
  +--------------------------------------------------------------------------------
  +Custom compilation options
  +--------------------------------------------------------------------------------
  +The list of custom compilation options is located [here](https://github.com/opcm/pcm/blob/master/CUSTOM-COMPILE-OPTIONS.md)
  