Update CONTRIBUTING.md
Update CONTRIBUTING.md

The list of main contributors was very outdated.

dnmvisser/simplesamlphp,simplesamlphp/simplesamlphp,simplesamlphp/simplesamlphp,ghalse/simplesamlphp,simplesamlphp/simplesamlphp,dnmvisser/simplesamlphp,ghalse/simplesamlphp,dnmvisser/simplesamlphp,ghalse/simplesamlphp,ghalse/simplesamlphp,dnmvisser/simplesamlphp,simplesamlphp/simplesamlphp