add token example to readme

Author: bobjflong

README.md

@@ -30,6 +30,9 @@ Using bundler:
 
 ```ruby
 intercom = Intercom::Client.new(app_id: 'my_app_id', api_key: 'my_api_key')
+
+# With an OAuth token:
+intercom = Intercom::Client.new(token: 'my_token')
 ```
 
 You can get your `app_id` from the URL when you're logged into Intercom (it's the alphanumeric just after `/apps/`) and your API key from the API keys integration settings page (under your app settings - integrations in Intercom).