Document needing zookeeper to run tests

Change-Id: Idf796844263097ec1543654c3f5a201060b3e4e8
diff --git a/TESTING.rst b/TESTING.rst
index 56f2fbb..f6d948e 100644
--- a/TESTING.rst
+++ b/TESTING.rst
@@ -17,6 +17,16 @@
 
   pip install tox
 
+As of zuul v3, a running zookeeper is required to execute tests.
+
+*Install zookeeper*::
+
+  [apt-get | yum] install zookeeperd
+
+*Start zookeeper*::
+
+  service zookeeper start
+
 Run The Tests
 -------------