You can add overlays using:

  lat: -12.043333,
  lng: -77.028333,
  content: '<div class="overlay">Lima</div>'

You must define latitude, longitude and the content of the map overlay.

NoteAlso, you must define a height to the content.

NoteAlso, you can define a verticalAlign, which can be top, middle or bottom, and horizontalAlign, which can be left, center or right.