Create a guide for migration from one instance setup to multiple instance setup

[yorugac]

Users of k6-operator can face migration issues where they need to re-configure both their setups and k6 scripts to distributed execution. Currently it's fully user's responsibility but a straight-forward guide on how to wrangle k6 executors into obedience without full understanding of their logic can be beneficial too. A related issue was raised in https://github.com/grafana/k6-operator/issues/90.

In general, this can be solved by re-computing number of VUs, e.g. if one was running X independent instances with Y VUs, for multiple instances setup they'll need X * Y VUs and parallelism set to X. This may be seen as basic arithmetic; however, a specific guide can both help people to operate between 1 instance VS N instances more easily and also serve as additional introduction into executors and execution segments.

Related issue: https://github.com/grafana/k6-operator/issues/94