make conf.py ready for RTD

docs/conf.py

@@ -54,3 +54,7 @@ html_theme = 'sphinx_rtd_theme'
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
+
+# Readthedocs.io needs us to tell it what the index file is. This defaults to
+# 'contents'
+master_doc = 'index'