doc: Describe the process to become a maintainer

Signed-off-by: Manuel Rüger <[email protected]>

Author: mrueg

MAINTAINER.md

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+# Maintaining kube-state-metrics
+
+kube-state-metrics is welcoming contributions from the community. If you are interested in intensifying your contributions and becoming a maintainer, this doc describes the necessary steps.
+
+As part of the Kubernetes project, we use the community membership process as described [here](https://github.com/kubernetes/community/blob/master/community-membership.md). We do not adhere strictly to the numbers of contributions and reviews. Still as becoming a maintainer is a trust-based process and we desire positive outcomes for the project, we look for a long-term interest and engagement.
+
+## Adding a new reviewer
+* Ensure the new reviewer is a member of the [kubernetes organization](https://github.com/kubernetes/org/blob/main/config/kubernetes/org.yaml).
+* Add the new reviewer to the [OWNERS](OWNERS) file to be able to review pull requests.
+* Add the new reviewer to the [kube-state-metrics-maintainers group](https://github.com/kubernetes/org/blob/main/config/kubernetes/sig-instrumentation/teams.yaml), to gain write access to the kube-state-metrics repository (e.g. for creating new releases).
+
+
+## Adding a new approver
+* Ensure the new approver is already a reviewer in the [OWNERS](OWNERS) file.
+* Add the new approver to the [OWNERS](OWNERS) file to be able to approve pull requests.
+* Add the new approver to the [SECURITY_CONTACTS](SECURITY_CONTACTS) file to be able to get notified on security related incidents.
+* Add the new approver to the [kube-state-metrics-admin group](https://github.com/kubernetes/org/blob/main/config/kubernetes/sig-instrumentation/teams.yaml), to get admin access to the kube-state-metrics repository.
+* Add the new approver to the k8s.io [OWNERS](https://github.com/kubernetes/k8s.io/blob/main/k8s.gcr.io/images/k8s-staging-kube-state-metrics/OWNERS) file to be able to approve image promotion from the staging registry.