title: Create an ID (row number) column
body: <p>I need to create a column with unique ID, basically add the row number as an own column.  My current data frame looks like this:</p>
  
  <pre><code>   V1  V2
  1  23  45
  2  45  45
  3  56  67
  </code></pre>
  
  <p>How to make it look like this:</p>
  
  <pre><code>V1  V2  V3
  1  23  45
  2  45  45
  3  56  67
  </code></pre>
  
  <p>?</p>
  
  <p>Many thanks</p>