title: What is Flow#join in Akka Streams
body: <p>I'm a learner of Akka Streams in Scala. While I was reading at the <code>IncomingConnection</code>, I found <code>Flow#join</code>. Then, I found the following image on the comment of <code>Flow#join</code>.</p>
  
  <pre><code>+------+        +-------+
  |      | ~Out~&gt; |       |
  | this |        | other |
  |      | &lt;~In~  |       |
  +------+        +-------+
  </code></pre>
  
  <p>But, I'm wondering what its structure is. I think the "join" makes a loop. </p>
  
  <p>So I would like you to explain what structure "join" makes, and tell me simple sample code using <code>Flow#join</code></p>