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/.gitignore b/.gitignore
index 228b0a8..1ce41cd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,6 +43,7 @@
 
 # Sphinx
 doc/build
+doc/source/tests
 
 # pbr generates these
 AUTHORS
@@ -54,4 +55,4 @@
 .*sw?
 
 # Files created by releasenotes build
-releasenotes/build
\ No newline at end of file
+releasenotes/build