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.
It's true, there are some "XBlocks" that are actually Django applications with XBlocks.  Those applications will need to have settings adjusted, but it is because they are full applications.  The XBlock documentation shouldn't discuss it.  If we need documentation for how to add new Django applications to the XBlock workbench, then let's make a separate section for that.  It should be a rare case.  We don't want people having to create Django applications.