title: pandas - multiple columns to "column name - value" columns
body: <p>I have a pandas dataframe with multiple columns and I want to "flatten" it to just two columns - one with column name and the other with values. E.g.</p>
  
  <pre><code>df1 = pd.DataFrame({'A':[1,2],'B':[2,3], 'C':[3,4]})
  </code></pre>
  
  <p>How can I convert it to look like:</p>
  
  <pre><code>df2 = pd.DataFrame({'column name': ['A','A','B','B','C','C'], 'value': [1,2,2,3,3,4]})
  </code></pre>