Mention R8 FAQ in Android example (#2075)

Author: Marcono1234

examples/android-proguard-example/README.md

@@ -7,3 +7,6 @@ access the fields of a class. It is necessary to configure ProGuard to make sure
 
 Also have a look at the [ProGuard manual](https://www.guardsquare.com/manual/configuration/usage#keepoverview)
 for more details on how ProGuard can be configured.
+
+The R8 code shrinker uses the same rule format as ProGuard, but there are differences between these two
+tools. Have a look at R8's Compatibility FAQ, and especially at the [Gson section](https://r8.googlesource.com/r8/+/refs/heads/main/compatibility-faq.md#gson).