title: inserting node in yaml with ruamel
body: <p>I would like to have printed the following layout: </p>
  
  <p><code>
  extra:
    identifiers:
        biotools: 
            - http://bio.tools/abyss
  </code></p>
  
  <p>I'm using this code to add nodes: </p>
  
  <p><code>
  yaml_file_content['extra']['identifiers'] = {}<br>
  yaml_file_content['extra']['identifiers']['biotools'] = ['- http://bio.tools/abyss']
  </code></p>
  
  <p>But, instead, I'm getting this output, that encapsulates the tool in []: </p>
  
  <p><code>
  extra:
    identifiers:
       biotools: ['- http://bio.tools/abyss']
  </code></p>
  
  <p>I have tried other combinations but didn't work? </p>