Document zuul client

Adds programoutput sphinx extension as a test dependency so doc
builds can include the program help text.

Change-Id: Iec2f09f710162614cbb393a5628204ddebe2e29f
diff --git a/doc/source/index.rst b/doc/source/index.rst
index e75e417..4b7b4b0 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -23,6 +23,7 @@
    launchers
    reporters
    zuul
+   client
    statsd
 
 Indices and tables