Docs/Readme have incorrect import instructions

[warmans]

The docs suggest importing the core module with a forRoot() call to pass the apiKey. But the sample application passes the key through a provider (and indeed forRoot is not applicable to the exported core module). Additionally it's not clear that to use google maps the google module must also be imported.

[nerd-cs]

@DaSchTour
From the documentation, it suggest to add apiKey in forRoot() NgMapsCoreModule.forRoot()
but I can't use forRoot() in app.module.ts

[warmans]

The sample application shows the correct way to import the module:

https://github.com/ng-maps/ng-maps/blob/master/projects/sample/src/app/app.module.ts#L25

[DaSchTour]

I updated the docs. Thanks