Update top-level README.md for PlayIntegrityAPI and add PlayIntegrityAPI top-level README.md.

Author: bkepez-go

PlayIntegrityAPI/README.md

@@ -1,6 +1,14 @@
 Android Security Samples Repository
 ===================================
 
+## [Play Integrity API End to End Sample](https://github.com/android/security-samples/tree/main/PlayIntegrityAPI)
+
+This sample demos a client and server setup, as well as step by step
+instructions for configuring the Google Play Console for testing of the Play
+Integrity API. This sample is meant to be followed as an End to End
+demonstration of the process for integrating the API, with 3 different
+micro-apps reflecting critical use cases.
+
 ## [Android Developer Verification APK Signing Example](https://github.com/android/security-samples/tree/main/AndroidDeveloperVerificationAPKSigningExample)
 
 This project shows you exactly which file to create (adi-registration.properties) and provides the necessary folder structure to guide you. Use this template to check your placement of the file and ensure the correct insertion of your unique developer 'snippet' for successful verification.
@@ -11,13 +19,13 @@ This project shows you exactly which file to create (adi-registration.properties
 > 
 > To find other samples that may be of interest, see [https://developer.android.com/samples](https://developer.android.com/samples).
 
-## [Android BiometricLogin Sample](https://github.com/android/security-samples/tree/master/BiometricLoginKotlin)
+## [Android BiometricLogin Sample](https://github.com/android/security-samples/tree/main/BiometricLoginKotlin)
 
 This is the canonical sample that demonstrates how to integrate Android Biometrics into your apps.
 Whether your project is for government, healthcare, financial, enterprise, or general usage, this
 sample should guide your implementation.
 
-## [Android DirectBoot Sample](https://github.com/android/security-samples/tree/master/DirectBoot)
+## [Android DirectBoot Sample](https://github.com/android/security-samples/tree/main/DirectBoot)
 
 This sample demonstrates how to store and access data in a device protected
 storage that is always available while the device is booted.