title: How to do git sub modules and why to use git sub module
body: <p>I have found an github project and it uses git submodules.</p>
  
  <p>What is the usage of git sub modules ?</p>
  
  <p>How to create git sub modules ?</p>
  
  <p>How they differ it from sub-tree ?</p>
  
  <p><a href="https://i.stack.imgur.com/R7qr8.png" rel="noreferrer"><img src="https://i.stack.imgur.com/R7qr8.png" alt="GitSubModule"></a></p>
  
  <p>Thanks </p>