title: Validating 'other' field only if dropdown value is 'other'
body: <p>Slightly complicated one to explain, but a very common use case.</p>
  
  <p>I have this  field on my page:</p>
  
  <pre><code>&lt;select class="form-control" name="how_you_heard_about_us" id="select-hearaboutus"&gt;
  </code></pre>
  
  <p>All of the options within it have names, including this one:</p>
  
  <pre><code>&lt;option value="Other"&gt;Other&lt;/option&gt;
  </code></pre>
  
  <p>I also have a text input, which is only required if the user selects 'Other'.</p>
  
  <p>Here's my condensed validation code:</p>
  
  <pre><code>$this-&gt;validate($request, [
      'first_name' =&gt; 'required|max:20',
      'last_name' =&gt; 'required|max:20',
      'how_you_heard_about_us' =&gt; 'required'
  ]);
  </code></pre>
  
  <p>I'm trying to set it up, so that the user is only required to enter into the text field, if the dropdown is set to 'other'.</p>
  
  <p>How can I conditionally require the free text field, in Laravel 5.2?</p>