Add usage example for `POST /api/v1/auth/token` in README

[huihsinhsiao]

I'd like to suggest a small improvement to the README.md.

During my testing, I wasn't sure what payload to provide and kept receiving the error: failed to decode PEM block containing public key.
This error message was a bit confusing. After digging into service/auth_svc_test.go, I realized that the endpoint actually requires a Private Key.

It would be very helpful, especially for those who are new to Go or just starting with this project, to add a brief example in the README showing how to call this API correctly.