readme: Add instructions on how to test XBlocks

Since most XBlocks has a similar way of testing them, including it in the workbench would reduce a lot of redundancy.
@dragonfi I can talk about this idea.  But I'm not sure what "custom setup file" you're referring to.  You shouldn't have to change INSTALLED_APPS in the workbench, because most XBlocks are not Django apps.  Maybe I don't understand?