docs(README.md): add toast options descriptions
docs(README.md): add toast options descriptions

* Thanks to @RangarajKaushikSundar 
* Original commit: acef5bb81bf252b76bdb5b5c02fb264b2f63e2e4
* Closes #59 
AllenFang/react-toastr,tomchentw/react-toastr,AllenFang/react-toastr