title: Empty Git submodule folder when repo cloned
body: <p>I have one repo hosted at <a href="https://github.com/aikiframework/json">https://github.com/aikiframework/json</a> . On my local copy, I added a submodule using the command</p>
  
  <pre><code>git submodule add git@github.com:jcubic/json-rpc.git json-rpc
  </code></pre>
  
  <p>Then I did a commit and push, and the changes appear on GitHub (I can click on it). But when I clone the repo</p>
  
  <pre><code>git clone https://github.com/aikiframework/json.git
  </code></pre>
  
  <p>the submodule folder json-rpc is empty.</p>
  
  <p>What am I missing here? Did I forget about something? Why is that folder empty?</p>