title: Just create a column with all days of one month - with pandas
body: <p>I really try for more than 4 hour to do a simple task:
  Create a column in a df with pandas that represent from day one to last day of the month.
  For example:</p>
  
  <pre><code>index date
  0     2018-08-01
  1     2018-08-02
  2     2018-08-03
  ...   ...
  </code></pre>
  
  <p>I'm giving up and do this in excel, save in csv to import again as a df in pandas...
  thanks for any help!</p>