title: correct usage of .gitignore (for file types)
body: <p>when I type git status I get the following message:</p>
  
  <pre><code>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)
  
      modified:   .DS_Store
      modified:   README.txt
      modified:   lib/PriceSeries.rb
  
  Untracked files:
    (use "git add &lt;file&gt;..." to include in what will be committed)
  
      MM8 Prices.pdf
      prices from EH.xlsx
  </code></pre>
  
  <p>I want to be able to say <code>git add .</code>, without adding the untracked files. I believe that the .gitignore file is supposed to be for this. My .gitignore file (in the .git folder) looks like this:</p>
  
  <pre><code>#Ignore all xlsx files
  .xlsx
  
  #Ignore all pdf file
  .pdf
  </code></pre>
  
  <p>What am I doing wrong?</p>