title: odoo make field readonly when not empty
body: <p>I have this field</p>
  <pre><code>&lt;field name=&quot;date&quot; /&gt;
  </code></pre>
  <p>and</p>
  <p><code>date = fields.Date('Order Date')</code></p>
  <p>How to make it readonly and can not be edited when there is value in it?</p>