Add .zuul.yaml to run java-prettify-build on proposed changes

Mirror the gitiles setup: register the java-prettify-build job via an
in-repo .zuul.yaml so that proposed java-prettify changes are verified
by Zuul -- built and tested standalone -- the same way gitiles uses its
own .zuul.yaml. java-prettify is a leaf project with no dependency on
gerrit-core, so it is built on its own rather than inside the Gerrit
tree.

Depends-On: https://gerrit-review.googlesource.com/c/zuul/jobs/+/621501
Issue: 302614311
Change-Id: Iea0c7dcfe1b49f5c4a9a609b213d89b1aff3e001

.zuul.yaml

diff --git a/.zuul.yaml b/.zuul.yaml
new file mode 100644
index 0000000..38b3104
--- /dev/null
+++ b/.zuul.yaml
@@ -0,0 +1,4 @@
+- project:
+    check:
+      jobs:
+        - java-prettify-build