add maintenance information

MAINTENANCE.md

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+# Moodle Recipe Maintenance
+
+All contributions should be made via a pull request. This is to ensure a certain quality / consistency, that others can rely on.
+
+
+## Maintainer Responsibilities
+
+A recipe maintainer has the following responsibilities:
+- respond to pull requests / issues within a week
+- make image security updates within a day
+- make image patch / minor updates within a week
+- make image major updates within a month
+
+In order to fullfill these responsibilities a recipe maintainer:
+- has to watch the repository (to get notifications)
+- needs to make sure renovate is configured properly
+
+## Merge rules
+
+A pull request can be merged if it is approved by at least one maintainer. For pull requests opened by a maintainer they need to be approved by another maintainer.
+
+## Becoming a maintainer
+
+Everyone can apply to be a recipe maintainer. Simply add your self to the list in the [README.md](./README.md) and open a new pull request with the change.