[feat]: Add Public Preview status note to RustShare README

[melisecolak]

Problem

Context

We want to make sure visitors set correct expectations when trying out RustShare.

A dedicated "Public Preview Status" section in the README helps prevent users from mistaking RustShare for a mature, production-ready system.

During the Public Preview launch, this wording is important because visitors may arrive from LinkedIn, X, GitHub search, email, or community posts.

Proposed Solution

Add a dedicated section to README.md titled:

Public Preview Status

RustShare is currently in Public Preview.

The project is early and actively evolving. We are opening the codebase now to collect technical feedback, invite early beta testers, and make it easier for first-time open-source contributors to help shape the direction.

Please do not treat this as a finished, production-ready, or enterprise-ready release.

Alternatives Considered

Leave the Public Preview note only in the README introduction, but a dedicated section makes the project status easier to find for first-time visitors.

Additional Context

This is a markdown-only documentation issue and is suitable for first-time contributors. No local Rust setup is required.

Checks

• I have searched existing issues and this feature has not been requested yet.