title: Merging and counting in dataframe
body: <p>I've got two pandas dataframe with unique and non-unique words:</p>
  <p>DataFrame 1:</p>
  <pre><code>Word Frequency Pos
  Dance 10       INFN
  Listen 5       INFN
  </code></pre>
  <p>And DataFrame 2:</p>
  <pre><code>Word Frequency Pos
  Dance 15       INFN
  Hear 5         INFN
  </code></pre>
  <p>So I need some how to merge these dataframes that rows like &quot;dance&quot; would have counted frequency from both dataframes, so output looks like this:</p>
  <pre><code>Word Frequency Pos
  Dance 25       INFN
  Hear  5         INFN
  Listen 5      INFN
  </code></pre>