title: add labels to polygons
body: <p><a href="https://stackoverflow.com/questions/38899190/geopandas-label-polygons">I have seen this question</a> where labels are added to polygons and they appear inside each polygon.
  I am trying to accomplish the same thing, but I am using a different format and can´t see how to apply the same method to my case.</p>
  
  <p>My polygons are like this:</p>
  
  <pre><code>import geopandas as gpd
  from shapely.geometry import Polygon
  boundary = gpd.GeoSeries({
      'foo': Polygon([(5, 5), (5, 13), (13, 13), (13, 5)]),
      'bar': Polygon([(20, 20), (40, 20), (40, 30), (20, 30)]),
  })
  
  
  boundary.plot(cmap="Greens")
  plt.show()
  </code></pre>
  
  <p>Any idea how to make each polygon have a label?</p>