add support for job timer

I'd like to run some jobs on daily basis and this PR add this
functionality.

Change-Id: I5ec5b736bda201c7f788b0c57d9ff4776c7ba631
diff --git a/README.rst b/README.rst
index b24514d..3438cb2 100644
--- a/README.rst
+++ b/README.rst
@@ -172,6 +172,8 @@
                 url: https://github.com/jenkinsci/docker
                 name: "Jenkins Docker Image"
             trigger:
+              timer:
+                spec: "H H * * *"
               github:
               pollscm:
                 spec: "H/15 * * * *"