Update the guidelines for contributions

CONTRIBUTING.md

@@ -1,45 +1,13 @@
 # Contributing
 
-## Local Setup
+This project is not looking for code contributions other than bug fixes. A
+(non-exhaustive) list of changes that will most probably not be merged contains:
 
-### Prerequisites
+* code optimizations - create an issue instead to discuss it
+* unsolicited feature additions
+* extensive code changes
 
-1. PHP 8.1.0 or higher.
-2. `composer` 2.4 or higher.
+In short: please discuss planned changes by opening a new issue.
 
-### Initial Setup
-
-```shell
-composer install
-composer install-tools
-```
-
-### Running the Tests
-
-The project is set up with PHPStan for static analysis as well as Unit tests and Integration tests.
-
-```shell
-# Run PHPStan and Unit Tests 
-composer test
-# Run PHPStan only
-vendor/bin/phpstan
-# Run Unit Tests only
-vendor/bin/phpunit --testsuite=unit
-# Run Integration Tests only
-vendor/bin/phpunit --testsuite=integration
-# Generate test coverage report into ./build/coverage
-composer test-coverage
-# Apply code style
-composer cs
-```
-
-The integration tests in this project require a working firebase project and a number of setup steps to pass locally.
-
-* Create a new [Firebase Project](https://console.firebase.google.com/).
-* Generate and download a private key for a service account to `tests/_fixtures/test_credentials.json`.
-* Create a Realtime Database and save its url as a JSON string to `tests/_fixtures/test_rtdb.json`.
-* Follow these [instructions](https://github.com/firebase/quickstart-js/tree/master/messaging) on how to generate a
-   Cloud Messaging device token and save the token as a JSON array to `tests/_fixtures/test_devices.json`.
-* Setup [multi tenancy](https://cloud.google.com/identity-platform/docs/multi-tenancy-quickstart) create a tenant, and
-   save its ID as a string to `tests/_fixtures/test_tenant.json`.
-* Enable Email/Password Sign-in method in Firebase Authentication.
+Support the project's development and keep it sustainable by becoming a
+[GitHub Sponsor](https://github.com/sponsors/jeromegamez).