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: I8f2df1f85345fea679c15d10f6eb0d92bd1c74b3

.zuul.yaml

diff --git a/.zuul.yaml b/.zuul.yaml
index e95b300..e55f00e 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-task-build
+    post:
+      jobs:
+        - plugins-task-build
+        - gerrit-plugin-publish:
+            dependencies:
+              - plugins-task-build