synchronize version table of indexrst with readme.rst (#669)

Author: Kyutatsu

docs/index.rst

@@ -40,7 +40,7 @@ This app supports the following combinations of Django and Python:
 1.11        2.7, 3.4, 3.5, 3.6, 3.7
 2.0         3.4, 3.5, 3.6, 3.7
 2.1         3.5, 3.6, 3.7
-2.2         3.5, 3.6, 3.7
+2.2         3.5, 3.6, 3.7, 3.8
 3.0         3.6, 3.7, 3.8
 ==========  =======================