docs: add HTTP API details to README

[Sheikh566]

Summary

This PR adds a new section to the README documenting a public HTTP API for goavatar.

Context

I initially planned to build a public identicon service in Go and discovered that goavatar was the perfect fit. To make the library accessible to non-Go developers and for quick
integration, I deployed it to Google Cloud Run. This update shares the public endpoint so others can easily use the tool via a REST interface.

[MuhammadSaim]

Thank you so much for this PR and for the thoughtful addition to the README 🙌

I really appreciate the time and care you put into documenting the public HTTP API and sharing the Cloud Run deployment.
Exposing goavatar via a public REST endpoint is a great idea and makes the project much more accessible to non-Go developers for quick integrations. This is a valuable contribution, and I’m grateful for your initiative and effort.

Thanks again for your precious work and for helping improve the project 🚀

[Sheikh566]

Thanks for merging. More contributions coming up.