title: How to validate string with Joi?
body: <p>I am using <code>Node Joi</code> for validation. I am new in node I want to validate <code>env</code> to accept only 2 words "Yes" or "No" What changes I have to make in the following code </p>
  
  <pre><code>schema = Joi.object().keys({
      app_id: Joi.string().required(),
      env: Joi.string().required()
  });
  </code></pre>