Updates to maintainership

[gavanderhoorn]

Due to internal changes at ABB CRC, @jontje will no longer be able to contribute to abb_libegm.

For now, I've taken over maintainership of this package, but only insofar as to handle the most urgent and breaking cases. Unfortunately I am not an expert EGM user, so I will not be in a position to continue development.

In #154, the support-level badges were already updated to better reflect the current status: abb_libegm is currently community supported. This means it does not get support from an OEM, nor from the ROS-Industrial consortia directly at this time. Maintenance and development will be on a best-effort basis and will depend on volunteers.

I'd like to take this opportunity to thank @jontje for his work on this package, his contributions to the ROS and ROS-Industrial communities and for supporting ROS users with ABB robots and controllers.