Add readonly plugin to the check pipeline

Change-Id: I873b49eba12e8ea05ab4f56c52cc714673ad20b7
diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index 7aa16f0..ac3b2fc 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -183,6 +183,12 @@
         - gerrit-plugin-build
 
 - project:
+    name: plugins/readonly
+    check:
+      jobs:
+        - gerrit-plugin-build
+
+- project:
     name: plugins/reviewers
     check:
       jobs: