Add link to documentation wiki in right marigin

The new entity based version of this project is great! (But it is not well documented.)

I am building a site where editors need to flush the cache table used to store compressed pages for anonymous users, but I don't want them to have access to other parts of the site configuration, and from reading the specs on the project page, this project looked like an exact match.

However, it took some experimentation to figure out exactly how to configure out what submodules to enable and how to configure them. The README tells you about all the wonderful things this project can do, but very little about how you configure the project for specific use ases.

I took the the time to note down the few steps that were required for my use case and to put it up as manpage in our documentation wiki - the URL to it is: https://www.drupal.org/node/2791075 →

THis page is pretty sparse now, but it is a start. Perhaps other that try out this project can fill it in with more use cases?

I also hope the project maintainer can put up a link to this documentation page as a documentation link in the right margin on the project page.