Add details of StoreKit configuration file setup?

[jameslavery]

I struggled to get products returned, until I found details of how to set up StoreKit testing here

would it be worth adding this to the README?

[vccosenza]

how did you get this set up with maui? did you need to bring the .storekit file into the ios platform folder anywhere or leave it in xcode? @jameslavery

[jameslavery]

The .storekit file should be left in xcode.

[nikmomo]

#6 has updated README, appreciated your contribution!