title: add CSS files to Phoenix
body: <p>I added a navbar.html.eex to my layout folder and this template is rendered from app.html.eex <code>&lt;%= render "navbar.html", conn: @conn, current_user: @current_user %&gt;</code></p>
  
  <p>in app.css:</p>
  
  <p><code>@import 'navbar.css';</code></p>
  
  <p>and it should point to my file: assets/css/navbar.css</p>
  
  <p>But the file does not load, in my browser console I get: <code>GET http://localhost:4000/css/navbar.css net::ERR_ABORTED</code> with a 404 error. </p>
  
  <p>I come from the Rails world and that's how I was used to do but maybe there is another way of doing it in Phoenix. I can't find a nice doc about managing css files in Phoenix so if anybody has some readings to recommend ! I'll be grateful ! </p>