Add cord-7.0 to the version selection popdown on master docs

Change-Id: I452f47cab370c1fa9e1a7db39c94a3a69de1f3e9
diff --git a/book.json b/book.json
index f832c23..c53b7fb 100644
--- a/book.json
+++ b/book.json
@@ -32,6 +32,10 @@
           "text": "Master (Devel)"
         },
         {
+          "value": "/cord-7.0",
+          "text": "7.0 (Stable)"
+        },
+        {
           "value": "/cord-6.1",
           "text": "6.1 (Stable)"
         },