Add description of how to actually use this great plugin

Thank you for making this plugin.

I had some trouble getting it to work with QGIS on my mac, so I wrote a little description of what I did to make it work: https://gist.github.com/AsgerPetersen/9ea79ae4139f4977c31dd6ede2297f90

As someone noticed on that GIST it would make sense to include the description with the plugin. You are welcome to copy from the text if you like.