Add Zuul postflight publish job

This was added by default for other non-core plugins in [1], but since
we define our own project, we need to add the job ourselves.

[1] https://gerrit-review.googlesource.com/c/zuul/config/+/314842

Change-Id: I298005bd93413a6a1a1f9a0282d7c9646a6ad648
diff --git a/.zuul.yaml b/.zuul.yaml
index 04cf314..448faaf 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -5,6 +5,15 @@
         tools/playbooks/install_docker.yaml
 
 - project:
+    description: |
+      Build the plugin in check, and also build and publish it after
+      every merged commit.
     check:
       jobs:
         - plugins-events-build
+    post:
+      jobs:
+        - plugins-events-build
+        - gerrit-plugin-publish:
+            dependencies:
+              - plugins-events-build