title: How to merge two dataframes based on missing criteria using dplyr?
body: <p>I have a dataframe like this</p>
  <pre><code>varA  varB  varC
    AA    45      
          32    D3
    DF          G5
  </code></pre>
  <p>And another like this:</p>
  <pre><code>varA  varB  varC
          45    F3
    BB          D3
    DF    22    
  </code></pre>
  <p>How could I merge these dataframes so the whole thing is put together into one?</p>
  <pre><code>varA  varB  varC
    AA    45    F3
    BB    32    D3
    DF    22    G5
  </code></pre>