Add doc for version list API

We can request version list by access endpoint root path, but this
is not mentioned in our webapi document.

Change-Id: I61d1b805f3bd83b5bc527ab84b3a36cca9a70f3e

doc/source/webapi/index.rst

@@ -7,3 +7,42 @@
 
    v1
    v2
+
+You can get API version list via request to endpoint root path. For example::
+
+  curl -H "X-AUTH-TOKEN: fa2ec18631f94039a5b9a8b4fe8f56ad" http://127.0.0.1:8777
+
+Sample response::
+
+  {
+      "versions": {
+          "values": [
+              {
+                  "id": "v2",
+                  "links": [
+                      {
+                          "href": "http://127.0.0.1:8777/v2",
+                          "rel": "self"
+                      },
+                      {
+                          "href": "http://docs.openstack.org/",
+                          "rel": "describedby",
+                          "type": "text/html"
+                      }
+                  ],
+                  "media-types": [
+                      {
+                          "base": "application/json",
+                          "type": "application/vnd.openstack.telemetry-v2+json"
+                      },
+                      {
+                          "base": "application/xml",
+                          "type": "application/vnd.openstack.telemetry-v2+xml"
+                      }
+                  ],
+                  "status": "stable",
+                  "updated": "2013-02-13T00:00:00Z"
+              }
+          ]
+      }
+  }