message: include a link to ENVVAR_README in the main README
diff: @@ -67,6 +67,12 @@ 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).
  +--------------------------------------------------------------------------------
  +PCM environment variables
  +--------------------------------------------------------------------------------
  +
  +The list of PCM environment variables is located [here](https://github.com/opcm/pcm/blob/master/ENVVAR_README.md)
  +
  --------------------------------------------------------------------------------
  Custom compilation options
  --------------------------------------------------------------------------------
  