Document when to use storekit 1, when to use storekit 2 implementation

[keesvandieren]

For iOS, therre are two implementations provided: one for Storekit 1, one for storekit 2

Update the README.md file in ios module to reflect when to choose what, and how to do it (Gradle dependencies, module configuration in the app it is being included on).

Include details from #273 in the documentation