Update module to be only Drupal 9 compatible
This updates version constraints to only allow Drupal 9 in the 2.x version. It also updates the README, and adds me as a maintainer
@nathandentzau what do you think of the format of the README re: the two versions?
Also are you happy with a 2.x
branch and we'll make that the default branch? And once this is done I'll tag a 2.0.0.