Adds instructions to run E2E tests (#83)

Author: JyotinderSingh

docs/contributing.md

@@ -47,11 +47,43 @@ Before submitting a pull request, please ensure your changes pass linting and un
   ```bash
   ruff check .
   ```
-- **Testing:** We use [Pytest](https://docs.pytest.org/) for unit tests. Run them with:
+- **Unit tests:** We use [Pytest](https://docs.pytest.org/) for unit tests. Run them with:
+
   ```bash
   pytest
   ```
 
+- **E2E tests:** End-to-end tests run real workloads against a GKE cluster. They live in `tests/e2e/` and are skipped by default unless explicitly enabled.
+
+  **Prerequisites:**
+  - A GCP project with a provisioned GKE cluster (see [Quick Start](../README.md#quick-start))
+  - Google Cloud SDK authenticated (`gcloud auth login` and `gcloud auth application-default login`)
+  - GKE credentials configured: `gcloud container clusters get-credentials <KERAS_REMOTE_CLUSTER> --zone <KERAS_REMOTE_ZONE> --project <KERAS_REMOTE_PROJECT>`
+  - Test dependencies installed: `pip install -e ".[test,cli]"`
+
+  **Required environment variables:**
+
+  | Variable               | Required | Default         | Description                    |
+  | ---------------------- | -------- | --------------- | ------------------------------ |
+  | `E2E_TESTS`            | Yes      | —               | Set to `1` to enable e2e tests |
+  | `KERAS_REMOTE_PROJECT` | Yes      | —               | Google Cloud project ID        |
+  | `KERAS_REMOTE_ZONE`    | No       | `us-central1-a` | GKE cluster zone               |
+  | `KERAS_REMOTE_CLUSTER` | No       | `keras-remote`  | GKE cluster name               |
+
+  **Run all e2e tests:**
+
+  ```bash
+  E2E_TESTS=1 KERAS_REMOTE_PROJECT=my-project python -m pytest tests/e2e/ -v -n auto
+  ```
+
+  **Run a specific test file:**
+
+  ```bash
+  E2E_TESTS=1 KERAS_REMOTE_PROJECT=my-project python -m pytest tests/e2e/cpu_execution_test.py -v
+  ```
+
+  Drop `-n auto` to run tests serially to make it easier to debug.
+
 ### Submitting changes
 
 1. Fork the repository