Add cloudspanner as documented supported db engine

Change-Id: I9ccd28c5bc3775aa0f4bd9db973f0a31fcb9dff7

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 95e060c..78c0d47 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -30,6 +30,7 @@
 * h2 (default)
 * postgresql
 * mysql
+* cloudspanner
 
 @PLUGIN@.localStorePath
 :    Specify the path to the directory in which to keep the back up database.