add 6.1 branch to menu in master

Change-Id: I1dbabed59b6d153656cb119cf8757dd0a6100c52
diff --git a/book.json b/book.json
index 83cd6ad..47ff947 100644
--- a/book.json
+++ b/book.json
@@ -30,6 +30,10 @@
           "text": "Master (Devel)"
         },
         {
+          "value": "/cord-6.1",
+          "text": "6.1 (Stable)"
+        },
+        {
           "value": "/cord-6.0",
           "text": "6.0 (Stable)"
         },