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.
@singingwolfboy While writing this, I was wondering if it would be possible to use `pkg_resources` to fill INSTALLED_APPS automatically from the installed XBlocks using something like this in `settings.py`:

    INSTALLED_APPS += tuple(e.name for e in pkg_resources.iter_entry_points('xblock.v1'))

Is there any pitfalls I'm not aware of? If you are not the right person, do you know who to talk this over?