title: SparkR data frame add column with constant value
body: <p>I want to do this in a sparkR data frame:</p>
  
  <p>I'm adding a column with the String "a" to the data frame</p>
  
  <pre><code>df$new_col &lt;- "a"
  </code></pre>
  
  <p>I can't find a way to do this in sparkR</p>