Summary:
The Android minSdk has been bumped in https://github.com/facebook/react-native/pull/38874 but not in the README.

## Changelog:
[General] [Fixed] - Updated docs to match Android 6.0 (API 23) minimum requirement.

Pull Request resolved: https://github.com/facebook/react-native/pull/42034

Test Plan: N/A

Reviewed By: fkgozali

Differential Revision: D52364522

Pulled By: arushikesarwani94

fbshipit-source-id: b04b5aa94b629380b559b2717e12a882f8817a6f