title: What does " -r " do in pip install -r requirements.txt
body: <p>I looked up how to install multiple packages from a requirements document using pip. The answers were mostly:</p>
  
  <pre><code> pip install -r requirements.txt
  </code></pre>
  
  <p>What does the <code>-r</code> do though? I can't find an answer for this and it isn't listed when I run <code>pip help</code>.</p>