docs(README.md): update to get a brief overview
docs(README.md): update to get a brief overview

* Added "Documentation" section. Ref #128

joshcarr/react-google-maps,jossi87/react-google-maps,RishabhJain96/react-google-maps,tomchentw/react-google-maps,ksavenkov/react-google-maps,jossi87/react-google-maps,RishabhJain96/react-google-maps