title: How to write a pandas.DataFrame to csv file with custom header?
body: <p>I have a dataframe</p>
  
  <pre class="lang-py prettyprint-override"><code>import pandas as pd
  df = pd.DataFrame([[1, 2], [3, 4]], columns=['a', 'b'])
  </code></pre>
  
  <p>I want to write <code>df</code> to a csv file but not using the columns <code>['a', 'b']</code>. The first line is my custom string and the rest are the content of <code>df.values</code>. For example:</p>
  
  <pre><code>numrows numcols note
  1 2
  3 4
  </code></pre>
  
  <p>Can I do this with pandas or I have to manually loop through the content and write to file?</p>