docs: restore ProGuard setup instructions

[arnabnandy7]

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Fixes #2089 ☕️

Summary

Restores the missing ProGuard section in the setup documentation using the existing rules from google-http-client-assembly/proguard-google-http-client.txt.

The documentation also separates the Android-specific warning suppression from the common rules, making the instructions applicable to desktop applications.

Validation

• git diff --check
• Documentation-only change; Java tests were not required

[ashughes]

The restored rules appear incomplete for strict R8 full mode: model construction also requires protection for public no-argument constructors.

I reproduced this with google-http-client and google-http-client-gson 2.2.0 using R8 9.4.24. With the rules proposed here, R8 makes a GenericJson response model abstract and removes its constructor. JSON parsing then fails in Types.newInstance() with NoSuchMethodException.

Adding the following rule alongside the existing attribute and annotated-field rules fixes the reproducer’s parsing cases, including typed nested models:

-keepclasseswithmembers,allowobfuscation class * {
    @com.google.api.client.util.Key <fields>;
    public <init>();
}

The existing annotated-field rule should remain: request classes with only parameterized constructors do not match this additional rule, and their JSON field names still need protection.

The 2.2.0 construction code calls Class.newInstance(), which requires a public no-argument constructor. These tests used JVM classfile output; I have not tested an Android APK using 2.2.0.

Could this documentation cover constructor retention too? Bundled consumer rules and an R8 regression test would also help prevent applications from having to discover this requirement independently.

[arnabnandy7]

Could this documentation cover constructor retention too? Bundled consumer rules and an R8 regression test would also help prevent applications from having to discover this requirement independently.

@ashughes yes why not, I've updated please review.