Add a document on how to run CI

[MRiabov]

Hello,
It would be useful to document on how to run CI with Genesis. It's not my first project, and CI genuinely takes 70% of my CI time - in a relatively greenfield project, CI of 25 minutes with 36 tests is unacceptable.

And I'm already using caches per test.

Add a doc... it doesn't quite look right. this is the biggest iteration blocker!

is it on me? yes. But I'm using genesis only occasionally - once a few months - and I can't remember all the ways how I should build my CI to work around the compile times.