title: Merge multiple rows into one using R
body: <p>Say I have this data frame:</p>
  
  <pre><code>Col1  Col2  
  ABC  Hello   
  ABC  Hi  
  ABC  Bye  
  </code></pre>
  
  <p>And want it like this:</p>
  
  <pre><code>Col1  Col2  
  ABC  Hello,Hi,Bye  
  </code></pre>