Document the test coverage workflow in README

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Author: Copilot

README.md

@@ -127,6 +127,26 @@ This SDK tracks the official [Copilot SDK](https://github.com/github/copilot-sdk
 - **VS Code Copilot Chat** — type `/agentic-merge-upstream`
 - **GitHub Copilot CLI** — use `copilot` CLI with the same skill reference
 
+### Automated Test Coverage Improvement
+
+The repository includes an [Agentic Workflow](https://github.com/github/gh-aw) that automatically identifies untested code and creates comprehensive tests to increase coverage. The workflow is triggered manually:
+
+**To run the workflow:**
+1. Go to the [Actions tab](../../actions/workflows/increase-test-coverage.lock.yml)
+2. Click "Run workflow"
+3. Optionally specify:
+   - `target_package` — Focus on a specific package (e.g., `com.github.copilot.sdk.json`)
+   - `min_coverage_threshold` — Target coverage percentage (default: 80%)
+
+**What it does:**
+- Analyzes current test coverage using JaCoCo
+- Identifies methods, branches, and edge cases lacking tests
+- Creates high-quality tests following repository patterns
+- Groups related tests logically
+- Creates a PR for human review with coverage metrics
+
+The workflow follows all repository conventions (Spotless formatting, existing test patterns, E2E test infrastructure) and ensures no production code is modified unnecessarily.
+
 ### Development Setup
 
 ```bash