polish README
This issue is about keeping the extension README in the way it will be seen - by the user of the extension. Possibly add some screenshots, too. In general: compare with https://github.com/WebFreak001/code-debug/blob/master/README.md and https://github.com/OlegKunitsyn/gnucobol-debug/blob/master/README.md and move the other parts out of the extension README to either the repo-README or other files.
Assigned to @jklowden because of the documentation and monitoring skills with the option to assign to @rdubner.