Document handling of optional dependencies

[nipafx]

Commit message comes later...

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PR checklist

The following checklist shall help the PR's author, the reviewers and maintainers to ensure the quality of this project.
It is based on our contributors guidelines, especially the "writing code" section.
It shall help to check for completion of the listed points.
If a point does not apply to the given PR's changes, the corresponding entry can be simply marked as done.

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Contributing

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I hereby agree to the terms of the JUnit Pioneer Contributor License Agreement.

[Michael1993]

Proposed commit message:

Document optional dependency policy #576 / #580

Update CONTRIBUTING.md with the new policy regarding optional dependencies in Pioneer.

Closes: #502
PR: #593

[nipafx]

Just a heads-up: I want to include much of the exploration done in #502 into the commit message, so please don't merge this yet.