docs: Update README with X-Verint-KAB-Original-URL header feature

- Add Source URL Header to features list
- Document /documents/{documentId} endpoint
- Add X-Verint-KAB-Original-URL header to response headers section
- Include curl examples for document export with header inspection
- Add documentation links for new feature spec and API contract

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -5,6 +5,8 @@ HTTP service that generates XML sitemaps listing all accessible documents in a G
 ## Features
 
 - **Sitemap Generation**: XML sitemap at `/sitemap.xml` listing all accessible Google Drive documents
+- **Document Export**: Export Google Drive documents with original source URL tracking
+- **Source URL Header**: X-Verint-KAB-Original-URL response header for content traceability
 - **RESTful URLs**: Document links in format `/documents/{documentId}` per sitemap protocol
 - **Service Account Auth**: JWT-based authentication using Google Service Account credentials
 - **Pagination Support**: Handles large document sets (up to 50,000 URLs per sitemap protocol)
@@ -64,6 +66,12 @@ curl http://localhost:3000/sitemap.xml | xmllint --noout -
 
 # Count documents in sitemap
 curl http://localhost:3000/sitemap.xml | grep -c '<loc>'
+
+# Export a document and view source URL header
+curl -I http://localhost:3000/documents/{documentId}
+
+# Export document and extract original Google Drive URL
+curl -D - http://localhost:3000/documents/{documentId} | grep X-Verint-KAB-Original-URL
 ```
 
 ## Architecture
@@ -162,6 +170,7 @@ Environment variables override JSON config (e.g., `PORT`, `GOOGLE_SERVICE_ACCOUN
 ### Endpoints
 
 - `GET /sitemap.xml` - XML sitemap of all accessible documents (200 OK with XML body)
+- `GET /documents/{documentId}` - Export Google Drive document with source URL tracking
 - `GET /*` - All other paths return 404 Not Found (empty body)
 
 ### Response Headers
@@ -171,6 +180,11 @@ Successful sitemap response (200 OK):
 - `X-Request-Id: req_<uuid>` - Request tracing ID
 - `X-Document-Count: <number>` - Number of documents in sitemap
 
+Successful document export response (200 OK):
+- `Content-Type: application/pdf` (or appropriate MIME type)
+- `X-Request-Id: req_<uuid>` - Request tracing ID
+- `X-Verint-KAB-Original-URL: https://drive.google.com/file/d/{fileId}` - Original Google Drive URL for content traceability
+
 ### Error Responses
 
 All errors return **HTTP status code only** with **no response body** (per specification):
@@ -272,7 +286,15 @@ ISC
 ## Documentation
 
 For detailed setup and usage instructions, see:
+
+### Sitemap Feature
 - [Quick Start Guide](specs/001-drive-proxy-adapter/quickstart.md)
 - [Feature Specification](specs/001-drive-proxy-adapter/spec.md)
 - [Implementation Plan](specs/001-drive-proxy-adapter/plan.md)
 - [Data Model](specs/001-drive-proxy-adapter/data-model.md)
+
+### Source URL Header Feature
+- [Quick Start Guide](specs/001-gdrive-url-header/quickstart.md)
+- [Feature Specification](specs/001-gdrive-url-header/spec.md)
+- [API Contract](specs/001-gdrive-url-header/contracts/response-headers.md)
+- [Implementation Plan](specs/001-gdrive-url-header/plan.md)