Update README.md with info on building and testing

The default MIDlet is specified by the midletClassName parameter, which can handle slash separators but can't handle a ".java" file extension. And RunTests is actually at the top level of the class hierarchy (inside the tests.jar file). So I would make this:

* Default *midletClassName* if none specified is `RunTests`

(I also applied some *name* and `value` style here; optional, but may be helpful to folks reading these docs.)