title: Strapi graphql mutation
body: <p>I'm using strapi as a headless CMS</p>
  <p>I've created a Customer content type and I've installed the graphql plugin.</p>
  <p>I'm trying to create a mutation in the graphql playground to create a new Customer</p>
  <pre><code>mutation createCustomer($username: String!, $email: String, $password: String){
    createCustomer(username: $username, email: $email, password: $password){
      _id username email password
    }
  }
  </code></pre>
  <p>I'm passing the username etc in the query variables</p>
  <pre><code>{ &quot;username&quot;: &quot;user1&quot; }
  { &quot;email&quot;: &quot;User1@test.com&quot; }
  {&quot;password&quot;:&quot;123456&quot;}   
  </code></pre>
  <p>I'm getting the error</p>
  <pre><code>&quot;message&quot;: &quot;Unknown argument \&quot;username\&quot; on field \&quot;createCustomer\&quot; of type \&quot;Mutation\&quot;.&quot;,
  </code></pre>
  <p>If I look in the Docs the it looks like.</p>
  <p><a href="https://i.stack.imgur.com/VJwFV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VJwFV.png" alt="enter image description here" /></a></p>
  <p>How can I write a mutation to create a new Customer</p>