include the possibility to embed videos in web page version of the paper

Once it will be implemented, we shall include into the doc that video embedding is possible. @agahkarakuzu kindly agreed to add an optional video_url in the frontmatter. So once [this issue](https://github.com/brainhack-proceedings/template/issues/5) will be closed, we can add it to the doc :) \\\ö// (but I am adding the note now so I don't forget later 🙃 )
The doc should include that the video has to be uploaded to a service which allows streaming, such as YouTube, Vimeo, or Youku.