Create .d.ts files for TypeScript

Hey,

thanks for the project! :100:
Sadly my CI currently fails as it expects the types declared.
```
Try `npm i --save-dev @types/react-responsive-masonry` if it exists or add a new declaration (.d.ts) file containing `declare module 'react-responsive-masonry';`
```

Now, I have never done this and would try to do it, however, if it an easy task for you I'd appreciate the help.
Let me know!