Add ggmap (longitude/latitude) examples to Spatial data page

[jtr13]

Explain that Google Maps now requires a credit card although they do offer $300 in free credit, and claim that they won't start charging without permission. (Presumably the service will stop working once the credit is used up if permission isn't granted to charge.)

Therefore, given this constraint, it is probably best to use get_stamenmap() examples

[Mrugankakarte]

I have added an example using get_stamenmap() with ggmaps. Kindly take a look at it. Made a PR for it. #250