Clarified roles in README.md

I tried to clarify the current roles in the project, explaining specifically what we (myself and @moosmannp) are supposed to do in the context of the GXFS project. @dsukhoroslov could you please check whether this is fine with you?

Signed-off-by: Christoph Lange (Fraunhofer FIT) christoph.lange-bever@fit.fraunhofer.de