title: Submit form only if at least one checkbox is checked
body: <p>i'm triyng to validate a form.
  In this form you've to choose at least one element by checkboxes, and I can't be sure about their quantity (it depends by elements number).
  I need to enable the submit input if one or more checkboxes are checked, and disable it if there aren't any checkbox checked, how can I do?
  Here's my code:</p>
  
  <pre><code>&lt;form id="booking"&gt;
  &lt;input type="checkbox" name="room1" class="roomselect"/&gt;
  &lt;input type="checkbox" name="room2" class="roomselect"/&gt;
  &lt;input type="checkbox" name="room3" class="roomselect"/&gt;
  &lt;input type="submit" value="Request" id="subm" /&gt;
  &lt;/form&gt;
  </code></pre>