V3: GET /v3 displays usage events endpoint

```json
      "usage_events": {
         "href": "https://api.example.com/v3/usage_events",
         "experimental": true
      },
```

[#169237857]

Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>
Co-authored-by: Brian Cunnie <bcunnie@pivotal.io>

Author: reidmit

app/controllers/v3/root_controller.rb

@@ -33,6 +33,7 @@ def v3_root
     links.merge!(create_link(:space_quotas))
     links.merge!(create_link(:stacks))
     links.merge!(create_link(:tasks))
+    links.merge!(create_link(:usage_events, experimental: true))
     links.merge!(create_link(:users))
 
     render :ok, json: MultiJson.dump({ links: links }, pretty: true)

spec/request/v3_root_spec.rb

@@ -94,6 +94,10 @@
           'tasks' => {
             'href' => "#{link_prefix}/v3/tasks"
           },
+          'usage_events' => {
+            'href' => "#{link_prefix}/v3/usage_events",
+            'experimental' => true
+          },
           'users' => {
             'href' => "#{link_prefix}/v3/users"
           }