title: git: stop warning me about changes in submodule
body: <p>I have a git submodule which is constantly showing "modified" in git status even though I never update it.</p>
  
  <pre><code># On branch master
  # Changes not staged for commit:
  #   (use "git add &lt;file&gt;..." to update what will be committed)
  #   (use "git checkout -- &lt;file&gt;..." to discard changes in working directory)
  #   (commit or discard the untracked or modified content in submodules)
  #
  #   modified:   MKNetworkKit (new commits, untracked content)
  #
  </code></pre>
  
  <p>How do I stop git from thinking about it?  I tried adding MKNetworkKit to .gitignore but it doesn't do anything.</p>