title: Git: how to exclude a subfolder in a repository?
body: <p>I have created a repository in a folder with my project, you know, <code>git init</code>, I have the Webstorm folder <code>.idea</code> and another one not related to this, I am not interested in keeping in the repository, and I don't want this folder to appear even as new files to add. 
  I want to forget about these subfolders.</p>
  
  <ol>
  <li>How can I exclude this subfolder before committing the whole repository?</li>
  <li>How can I exclude this subfolder after committing the whole repository?</li>
  </ol>