Clarify that setting request-merge-queue needs the user to be in contributors.json

[pulkomandy]

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Clarify that setting request-merge-queue needs the user to be in contributors.json

This is not mentionned anywhere, and it is not immediately obvious how to get permission to set labels.

[pulkomandy]

Hello,
It seems that adding myself to contributors.json allows me to set labels in pull requests for the main repository, but not here in the documentation repository. Is this intentional? What is the next step of the merging process for the documentation repository?