[Janitorial] Please add MAINTAINERS.md

[RichiH]

Is your feature request related to a problem ?

I'm going through all of prometheus-community/ and adding an issue to create a MAINTAINERS.md where it's missing.

Ideally model it on https://github.com/prometheus/prometheus/blob/main/MAINTAINERS.md with a split between general mantainers and subsystem maintainers if applicable.

Thanks,
Richard

Describe the solution you'd like.

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Describe alternatives you've considered.

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Additional context.

No response