[DOC] Update documentation for MTF

[phracek]

This issue should update meta-test-family.readthedocs.io with this sort of information.

I would prefer to create new section, called 'examples', in http://meta-test-family.readthedocs.io/en/latest/user_guide/index.html

• How to specify various asserts in the tests
• Provide a way, every tests should be run-able standalone
• Provide examples, how to handle with testing multi-container setup-up, we can kill and start new containers easily
• Document, how to run either any (even) local image with various variables
• Devel should be able to run all the tests for a specific image using one command

[jscotka]

Do you think that it make sense to copy UnitTest specification here.
I prefer to add link, that avocado is unittest and link to unittest docs for asserts

[phracek]

Yeah, links would be enough for now.