Add obofoundry topic to repository

[cthoyt]

The OBO Foundry has recently created a topics page at https://github.com/topics/obofoundry. It would be great if you could add the obofoundry topic to your repository, since it's currently being considered for the OBO Foundry. If you're not sure how to do that, follow the instructions here. The main issue for this task is at OBOFoundry/OBOFoundry.github.io#1538 in case you want some more context or to join the larger discussion.