Streams Contributors guide

Let's compile a minidoc for contributors on when, how and where to add new combinators. There are many questions here that needs at least some fuzzy initial guidance:

what combinators do we accept? Of course this is not definable, but sometimes it is easier to show some counterexamples that are trivially no-go if a combinator is to be accepted, where should it go? I.e. FlowOps, Source, Sink or external object if accepted and goes into the FlowOps/Source/Sink APIs then describe all the places where modifications needs to be done establish a guideline for testing these and documenting these