title: merge two dataframe columns into 1 in pandas
body: <p>I have 2 columns in my data frame and I need to merge it into 1 single column</p>
  
  <pre><code>Index  A             Index   B
  0      A             0       NAN
  1      NAN           1       D
  2      B             2       NAN
  3      NAN           3       E
  4      C             4       NAN
  </code></pre>
  
  <p>there will always be one 1 value across the Columns, i want my result to look like</p>
  
  <pre><code>0    A
  1    B
  2    C
  3    D
  4    E
  </code></pre>