update links to old versions of doc to include 2.1

Author: markxnelson

site/README.md

@@ -13,5 +13,6 @@ If you wish to view documentation for an older version, please see:
   Both versions are archive here:
     * [Old Version 2.0.1](v2.0.1)
     * [New Version 2.0.1](https://oracle.github.io/weblogic-kubernetes-operator/2.0.1)
+* [Version 2.1](https://oracle.github.io/weblogic-kubernetes-operator/2.1)
 
 For a guide to the operator project, refer to this [README](../README.md).