title: Identify unique user in Dialogflow V1
body: <p>So I am testing out Dialogflow and one of the first questions I have is: how does my bot know who it is talking to? I need to identify a user and keep that information for as long as I can. The basic scenario being:</p>
  
  <ol>
  <li>User starts his/her first conversation.</li>
  <li>Chatbot send a fulfillment request to the server trying to match a user within its own database.</li>
  <li>The user is found, the information (as a JWT or some other token) is sent back to Dialogflow and stored there for further communication. In reality, this part would involve asking for user email, sending a verification code to that email and then verifying the user with the code.</li>
  <li>User then starts chatting with a bot and all fulfillment requests get the unique token stored for this very user, so that my REST API knows which user is being served with the response.</li>
  </ol>
  
  <p>Couldn't find anything about it in the docs (maybe I am looking in the wrong places).</p>
  
  <p>There will be several integrations, like Messenger, Viber, Telegram. I dunno, maybe those APIs add some unique information on the user?...
  Thanks for the help!</p>