Need to have proper permissions settings.

cf https://github.com/nf-core/dualrnaseq/pull/3

This PR was merge (there was actually enough agreements, but I still think it as been merged) by someone who is on slack but not on the channel for this pipeline and who I haven't seen any comment on the PR in question either.
Not sure of the current involvement in the project.
From what I understood, this issue has happened already with the kmermaid pipeline too.
So I really think that stricter permissions settings should be enforced especially for merging PR.
I'm tagging the @nf-core/core team for now, but any idea is welcome.

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Start by reviewing the linked nf-core/dualrnaseq pull request and the issue's reference to the kmermaid pipeline. Identify the current permission and merge settings, then define and document stricter enforcement with the nf-core/core team; done means the agreed permissions prevent unauthorized merges.