Document Typescript casting of event payload

[Falke-Design]

In the event payload the return type of layer is `L.Layer? so it needs to be casted to the Layer-Type.

layer.on('pm:edit', (e) => {
  if(e.shape === 'Polygon'){
     (e.layer as Polygon).getLatLngs();
  }
});

I think following should work too: (But not tested)

layer.on('pm:edit', (e) => {
  if(e.layer instanceof L.Polygon){
     (e.layer as Polygon).getLatLngs();
  }
});

This should documented in the Readme

Explainations:

• https://stackoverflow.com/questions/69621270/how-to-get-latlngs-values-after-edit-a-polygon-in-leaflet-geoman/69626304#69626304
• Error trying to access layer attributes with Typescript #945