Link base-demo reference project from README

[codeforester]

Summary

Add a direct link from the Base README to the public codeforester/base-demo reference project and show the shortest clone/setup/demo flow.

Motivation

Base's focused docs already describe base-demo, but the top-level README is the product front door. New readers should be able to discover the reference project without digging into the docs map.

Acceptance Criteria

• The README links to https://github.com/codeforester/base-demo.
• The README shows a concise example flow for cloning, setting up, and running the base-demo walkthrough.
• The change stays docs-only.
• git diff --check passes.