chore(docs): Update README

[Jyyjy]

Copied from https://github.com/apache/flagon-useralejs/issues/505

Problem

Our README is stale, has broken links (ex: "What's New" in the TOC), and is not sufficient for newcomers to immediately get off the ground without running into issues.

Solution

Remove stale parts
Update links
Make link to CONTRIBUTING.md and who should go there obvious