Add documentation around why / how react-native-typescript-transformer works

It seems like most templates / tutorials nowadays with react-native + typescript suggest to use this package without much explanation about why / how it works - which is fine, but without much explanation besides "Seamlessly use TypeScript with react-native >= 0.45", it took a bit of digging to figure out how this actually works.

I found these two answers helpful to actually explain what was going on - it might be useful to just incorporate some of the content here into the README docs:
- https://blog.novanet.no/easy-typescript-with-react-native/
- https://stackoverflow.com/a/51758597/3307657