Improve TradeMe repository README

[mrfloris]

Summary

• replace the one-line README with a concise introduction to TradeMe
• clarify that this repository currently serves as the public issue and feature-request tracker and does not contain source or downloads
• link the official TradeMe resource, CMILib dependency, issue tracker, and Zrips website
• add practical bug-reporting guidance and direct sensitive exploit reports to private contact

Why

The repository is actively used for TradeMe reports, but its current README does not explain the plugin, the repository's observed role, its CMILib requirement, or where users should obtain documentation and downloads.

The README intentionally leaves changing compatibility ranges, commands, permissions, and API details on the official resource page rather than duplicating information that could drift.

Validation

• reviewed every product and dependency claim against the repository's contents and issue usage, maintainer-authored SpigotMC information, and the official CMILib resource
• confirmed that the upstream branch still contains only .gitignore and the original one-line README
• ran git diff --check
• changed only README.md; no plugin source, build, or runtime behavior is affected