title: What does :submodules => true mean in a CocoaPod Podspec?
body: <p><strong>TL ; DR :</strong></p>
  
  <p>What does <code>CocoaPods</code> do if you set <code>:submodules</code> (part of <code>spec.source</code>) as <code>true</code>?</p>
  
  <p><strong>Full Version:</strong></p>
  
  <p>My previous project structure looked like</p>
  
  <pre><code>Project
  Linked To --&gt; Core static library (submodule added as a subproject)
              --&gt; Linked To --&gt; several dependent submodules (files added within subproject)
  </code></pre>
  
  <p>I'm now in the process of transitioning to <code>CocoaPods</code> for dependency management.</p>
  
  <p>My new (transitional) structure looks like</p>
  
  <pre><code>Project
  --&gt; Depends on Core CocoaPod and few other CocoaPods
       --&gt; Core depends on several open-source libraries
       --&gt; Core also depends on a few submodules (in process of converting these)
  </code></pre>
  
  <p>I've seen other projects (e.g. <a href="https://github.com/CocoaPods/Specs/blob/master/AFNetworking/2.1.0/AFNetworking.podspec" rel="noreferrer">AFNetworking</a>) specify their <code>source</code> like this:</p>
  
  <p><code>s.source   = { :git =&gt; 'https://github.com/AFNetworking/AFNetworking.git', :tag =&gt; "2.1.0", :submodules =&gt; true }</code></p>
  
  <p>What does <code>CocoaPods</code> do if you pass <code>:submodules</code> as <code>true</code> as in this example? Is this the right setting for this use case?</p>
  
  <p>I can't seem to find this documented anywhere on <a href="http://cocoapods.org/" rel="noreferrer">CocoaPods.org</a>.</p>