Optimize README: improve structure, add chain table, and contact info

[GalaxySciTech]

Summary

Overhauls the README.md to be more professional, scannable, and informative.

Changes

• Header — Added a one-line tagline beneath the project name and a JitPack badge alongside the existing CI/issues/PR/contributors/license badges.
• Navigation — Added a quick-links bar (Supported Chains · Quick Start · Integration · Offline Signing · Contact) for fast in-page navigation.
• Introduction — Rewrote to clearly describe what Tokencore does (HD wallet derivation, encrypted keystore management, offline signing) and who it's for.
• Supported Chains — Replaced the bullet list with a structured table showing chain names, supported token standards, and key features.
• Quick Start — Streamlined the code examples with numbered steps and removed redundant boilerplate.
• Offline Signing — Reorganized Bitcoin/Ethereum/TRON examples with inline comments for clarity.
• Build & Test — Consolidated into one clean section.
• Project Structure — Added a source tree diagram so contributors can orient themselves quickly.
• Contact — Added Telegram (t.me/GalaxySciTech), kept the website link, and added a direct link to GitHub Issues.
• Disclaimer — Moved the learning-purpose note to a clearly marked disclaimer at the bottom.