Set something that rtd.org expects

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
diff --git a/source/conf.py b/source/conf.py
index 515ffde..5b677b9 100644
--- a/source/conf.py
+++ b/source/conf.py
@@ -53,3 +53,5 @@
 
 # Don't highlight by default
 highlight_language = 'none'
+
+master_doc = 'index'