Explain how to wire up ci.nix

[domenkozar]

pull in dimension from https://github.com/hercules-ci/hie-core-nix/pull/4/files

[michaelpj]

As a bonus, it would be nice to tell people how to import ci.nix into a release.nix if they still have one.

(Answer: you just import it, but you have to strip out all the recurseForDerivations otherwise Hydra gets unhappy that they're not jobs.)

[roberth]

I've previously added the hydra bit to https://docs.hercules-ci.com/hercules-ci/reference/evaluation/#_differences_with_hydra but not yet to the repo setup guide.