Build stable-3.0 branch of readonly plugin

Change-Id: I56d35675609abdd95c0cf7cde42ce1ee9f142693
diff --git a/jenkins/gerrit-bazel-plugin-readonly.yaml b/jenkins/gerrit-bazel-plugin-readonly.yaml
index 69f019d..6f64f97 100644
--- a/jenkins/gerrit-bazel-plugin-readonly.yaml
+++ b/jenkins/gerrit-bazel-plugin-readonly.yaml
@@ -4,4 +4,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.0
             - stable-2.16