Add docs section about mixins

[swistakm]

Usage of mixin classes may be a bit confusing. Extend the guide/resources.rst to provide introduction to mixins and how to use them with BaseResource class.

Additional tasks related with this issue:

• If initial support of optional contexts (refs #32) #33 gets merged, add some brief information about context-aware classes and point to proper document for more detailed description on this topic.
• Decide on how to properly call all the retrieve/update/list/etc. methods and do cleanup in the docs to unify this terminology
• Put a small glossary for the users and doc writers somewhere