I have installed your plugin, but it doesn’t seem to work… 
I’ve followed instruction here Install Plugins in Discourse
After installation Discourse works fine, but it seems nothing changed: if I look at users’ profiles of the group I wanted to force to watch a category, nothing changed, the category doesn’t appear among the watched ones.
This is my fork, maybe I did some mistakes…?
https://github.com/don-durito/discourse-watch-category-mcneel/blob/master/plugin.rb