Add an error if a forRoot method is used not in a root module

[ddubrava]

🚀 Feature Request

✍️ Description

forRoot must be used only in a root module, check this article - https://angular.io/guide/singleton-services#the-forroot-pattern. So we should add an error if it's imported in children modules.

[ddubrava]

We can't add an error, it seems it's not possible to determine when forRoot is used outside of a root module. We can only determine when it's used twice but it's not the solution.

Add some documentation changes here - https://github.com/ddubrava/angular8-yandex-maps/commit/4e1a082d46a58495a567e46ca335b992c71c0c53