title: AWS S3 configure sub-domain
body: <p>I'm using Amazon's simple storage service (S3). I noticed that others like Trello were able to configure sub-domain for their S3 links. In the following link they have <code>trello-attachments</code> as sub-domain. </p>
  
  <p><code>https://trello-attachments.s3.amazonaws.com/.../.../..../file.png</code></p>
  
  <p>Where can I configure this? </p>