Document how to configure `haskell-ci`

[konsumlamm]

Please mention cabal.haskell-ci and the CLI options in the README.

I only found out that you can have a cabal.haskell-ci file with configurations because I saw containers use it and I only found out that this repo has a documented cabal.hasell-ci file through #274. It also too me a while to try running haskell-ci --help to see all the CLI options (because the README makes no mention of other options).