update README

For one the readme still mentions ralf as contact person...

Connect to https://gitlab.iap.kit.edu register yourself and join the "Air Shower Physics" group. Write to me (ralf.ulrich@kit.edu) only in case there are problems with that.

Do we have a webpage that lists the people in the steering committee? We could point there in case of problems.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information