List Prometheus v3 coordinators in MAINTAINERS.md

The Prometheus v3 coordinators are full members of the Prometheus GH
org, a status that our governance specifies for maintainers. As
discussed, it appears best to formalize maintainership by listing the
coordinators in the MAINTAINERS.md file of prometheus/prometheus.

Signed-off-by: beorn7 <beorn@grafana.com>

MAINTAINERS.md

@@ -30,3 +30,13 @@ size of this repository, the natural changes in focus of maintainers over time,
 and nuances of where particular features live, this list will always be
 incomplete and out of date. However the listed maintainer(s) should be able to
 direct a PR/question to the right person.
+
+v3 release coordinators:
+* Alex Greenbank (<alex.greenbank@grafana.com> / @alexgreenbank)
+* Carrie Edwards (<carrie.edwards@grafana.com> / @carrieedwards)
+* Fiona Liao (<fiona.liao@grafana.com> / @fionaliao)
+* Jan Fajerski (<github@fajerski.name> / @jan--f)
+* Jesús Vázquez (<jesus.vazquez@grafana.com> / @jesusvazquez)
+* Nico Pazos (<nicolas.pazos-mendez@grafana.com> / @npazosmendez)
+* Owen Williams (<owen.williams@grafana.com> / @ywwg)
+* Tom Braack (<me@shorez.de> / @sh0rez)