docs: add Express Router upload example

[vibhor-aggr]

Summary

Adds a concise README example showing Multer middleware used with an express.Router() route.

This addresses the confusion raised in #596 by showing that the same upload.array(...) middleware pattern used on app.post(...) can be mounted directly on a router route.

Fixes #596.

Verification

• npm test (80 passing)
• npm run lint
• git diff --check