Add test documentation for docs tox target

Enable sphinx to generate documentation from docstrings by running
'tox -e docs'.

Change-Id: I5996e5f07493f69f14172b4bb0535852e89d5456
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 91388a9..e69a4ec 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -15,6 +15,7 @@
    readme
    installation
    contributing
+   test_descriptions
 
 Indices and tables
 ==================