Add link to Zuul v3 docs to the README

Put it in up high so that people can see it.

Change-Id: Ib80a636330749a4ef6c7327fde22293d7f54155e
diff --git a/README.rst b/README.rst
index 16e7385..52b89df 100644
--- a/README.rst
+++ b/README.rst
@@ -7,6 +7,9 @@
 preparation for the third major version of Zuul.  We call this effort
 `Zuul v3`_ and it is described in more detail below.
 
+The latest documentation for Zuul v3 is published at:
+https://docs.openstack.org/infra/zuul/feature/zuulv3/
+
 Contributing
 ------------