title: How to create submodule in existing repo
body: <p>I'm beginner to git &amp; I have the following folder structure for new project, so what I did is I went to visual studio and I made empty project it  has this folder structure:</p>
  <pre><code>project: (Repo) 
  folder1 (sub1)
  folder2 (sub2)
  </code></pre>
  <p>I'm using git locally.<br />
  I created a repo for project using sourcetree,<br />
  now I cannot add any submodule, I don't know why.</p>
  <p>In powershell whenever I try:</p>
  <pre><code>PS E:\Projects\Project&gt; git submodule add ./sub1
  </code></pre>
  <p>I get the following error:</p>
  <pre><code>sub1 already exists in the index
  </code></pre>
  <p>then I decided to remove sub1 using <code>git rm -r sub1</code>,and add it again, then when I tried to add it again using <code>git submodule add</code> again, I received this new error:</p>
  <pre><code>sub1 already exists and is not a valid git repo
  </code></pre>
  <p>So what I'm doing wrong?</p>