Document becoming a maintainer

This process has been the case since we took over maintainership of the
rules, but it hasn't been written down. We want this on ramp to be clear
in the case that folks are interested.

MAINTENANCE.md

@@ -11,6 +11,21 @@ rules repos (this doc is not duplicated across the repos):
 The current maintainers can be seen in the [CODEOWNERS](CODEOWNERS)
 file.
 
+## How to become a maintainer
+
+These rules are community maintained. Any community member can become a
+maintainer. Ideally we have a large set of maintainers who can work on
+improving different parts of the rules for different use cases. In order
+to become a maintainer you must demonstrate proficiency working on these
+bazel rules. The best way to do this is to contribute multiple
+non-trivial features over time. If you would like to become a
+maintainer, once you have a collection of non-trivial changes, reach out
+to Keith Smiley ([email protected]) with links to your changes.
+Once you have met these criteria the existing maintainers will vote and
+require a 2/3rds majority to add new maintainers. Otherwise the
+maintainers will provide specific feedback on what they would like to
+see before granting access.
+
 # Upstream changes
 
 While these rulesets are now maintained outside of Google, Google still