IntelliJ setup instructions

https://github.com/google/error-prone/wiki/For-Developers talks about code style but not about importing the project as a whole.

https://groups.google.com/g/error-prone-discuss/c/W-53qb8jwhE mentions the possibility of including `.iml` files in the repo. My understanding is that a "full" setup requires more than that, but then my understanding is that IntelliJ is supposed to do a lot of things automatically when you import the project, and that has not been my experience :( So it's possible that something deeper is wrong with my installation.