[google_sign_in] Add contributor setup instructions (#9489)

Adds a CONTRIBUTING.md that explains how to configure the example app for manual testing, since we don't currently maintain a central project for testing it.

Also opportunistically fixes a minor latent mistake in an older CHANGELOG entry, so that the next publication will include that fix.

Fixes flutter/flutter#83759
Fixes flutter/flutter#167906

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in/CHANGELOG.md

@@ -13,7 +13,7 @@
 
 * Adds a sign-in field to allow Android clients to explicitly specify an account name. This
   capability is only available within Android for the underlying libraries.
-* Updates minimum supported SDK version to Flutter 3.22/Dart 3.4.
+* Updates minimum supported SDK version to Flutter 3.27/Dart 3.6.
 
 ## 6.2.2
 

packages/google_sign_in/google_sign_in/CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# App configuration
+
+Manual end-to-end testing of `google_sign_in` requires configuring the example
+app with an active server-side GCP project. The example app currently has
+placeholder information (in files such as Info.plist and google-services.json)
+corresponding to a project that no longer exists, and there is currently no
+active, team-managed test project.
+
+Before running the example app, follow the
+[README's platform integration instructions](./README.md#platform-integration)
+for the platform(s) you are testing.