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.
@nedbat @antoviaque Sorry, I was under the impression that most XBlocks work this way. (At least the ones I had to work with did. Talk about generalizing from a small sample...)

It's still worth considering, as that would automate the inclusion of those XBlocks that are apps. (Unless they are really rare of course.)