Document minimum Android SDK version and iOS build target

**Is your feature request related to a problem? Please describe.**

The plugin fails to build on Android unless:

* `compileSdkVersion` >= 32
* `minSdkVersion` >= 22
* `targetSdkVersion` >= 32

and fails to build on iOS unless:

* minimum deployment target >= 13.0 (which needs to be set in the Podfile and XCode config)

**Describe the solution you'd like**

It would be really helpful if this was documented in the README.

Start with the README and add the Android compileSdkVersion, minSdkVersion, and targetSdkVersion requirements, along with the iOS minimum deployment target and the Podfile and Xcode configuration locations. The work is done when all stated version thresholds and configuration locations are documented clearly.

Written by the indexing model from the issue text.