Build readonly against gerrit stable-3.6

Bug: Issue 15831
Change-Id: I2112ef9bf99c1c9e91e199a430b0eee1a2bca5d0
diff --git a/jenkins/gerrit-bazel-plugin-readonly.yaml b/jenkins/gerrit-bazel-plugin-readonly.yaml
index c30eb3c..b71d37e 100644
--- a/jenkins/gerrit-bazel-plugin-readonly.yaml
+++ b/jenkins/gerrit-bazel-plugin-readonly.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3