Explanation of what needs to be changed in the code in some unusall cases.

Hi Sergei!

I have run into some problems when using translated slugs of related objects with your plugin. I have added some explanations to README and the class of what needs to be changed in order for the plugin to work in such situations. 

I don't think a generic solution for this problem is possible that's why I have just added a guide of what to do in such a case.

Cheers,
Michal