Update documentation to include examples of ES6-style imports (`import * as .... from 'esri-leaflet-geocoder'`)

I installed this project via npm as well as it's types for my Gatsby (react) project, but I am unable to use it thru the leaflet package as the documentation suggests.

I ran `npm i esri-leaflet-geocoder --save` and `npm i @types/esri-leaflet-geocoder --save` which installed properly, but when I go to use the geocode like the documentation suggests:

```
L.esri.Geocoder.reverseGeocode(...
```

I get a runtime error that `Geocoder` is undefined.

I can get around this by importing the package directly:

```
import * as ELG from 'esri-leaflet-geocoder'

ELG.reverseGeocode(....
```

However, when I do it this way. I run into issues with the api key that I've opened another issue about