Add leaflet to dependencies

[samehraban]

I suppose this lib has a dependency to leaflet v1.0+. I had a lot of trouble make this work with version 0.7 and repeatedly got error:

TypeError: Cannot read property 'getRenderer' of undefined

please add specific leaflet version to dependencies or state the version this lib supposed to work alongside.

[yohanboniface]

Yes.
Can you make a PR to add this to the README?

[yohanboniface]

Fixed with #3.