"Supported features" list

[yorugac]

There were situations when adding new functionality to the operator broke previously supported use cases. This problem happens mostly due to two main reasons:

• time limit: not enough grokking, not enough testing of the PR, etc.
• lack of knowledge: people are using k6-operator in multiple ways and it is meant to be used this way. But it's not always easy to predict how given change will impact all those different scenarios, esp. when the scenarios are not even known.

Proposed solution: create a list of "officially supported features of k6-operator" and refer to it when in doubt. Such a list can also be public, to set clear expectations to the community.
As a bonus, this would also show the way to how to create an exhaustive suite of functional tests that will be checking all those features.