Add CONTRIBUTING.md

[yorugac]

Feature Description

Contributors in k6-operator has been finding their own path, and with a very good success rate, IMHO. But recently it has been raised a few times that a contributor guide would be helpful.
It's also quite noticeable that the Helm support is adding up to complexity.

Suggested Solution (optional)

Some initial thoughts:

• it is preferable to have an issue and some discussion prior to opening a PR
• move out development section from README.md to CONTRIBUTING.md and expand it as necessary
• describe how PRs are tested
• maybe make a separation of different types of PR. Case in study: Allow multiple replicas to be configured via helm chart #328

Already existing or connected issues / PRs (optional)

No response