Added .d.ts file, so lib can be used in typescript react-native projects

[kleydon]

Hi,

Thanks for creating this library.

In this PR, I've added a .d.ts file, so that:

• The library can be used in typescript react-native projects
• There is auto-complete and type-checking (in VS Code, within typescript projects anyway)

Looking forward to your review,

K

[kleydon]

NOTE: I've tested this in the context of a typescript react-native app; so far, so good.