Doc: update the README.md

[tuhaihe]

Change logs

Update the description and information in README.md to give everyone a better understanding of the Cloudberry Database.

Why are the changes needed?

Update the file to catch up with the status.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

No test is needed.

Contributor's Checklist

Here are some reminders and checklists before/when submitting your pull request, please check them:

• Make sure your Pull Request has a clear title and commit message. You can take git-commit template as a reference.
• Sign the Contributor License Agreement as prompted for your first-time contribution(One-time setup).
• Learn the coding contribution guide, including our code conventions, workflow and more.
• List your communication in the GitHub Issues or Discussions (if has or needed).
• Document changes.
• Add tests for the change
• Pass make installcheck
• Pass make -C src/test installcheck-cbdb-parallel
• Feel free to request cloudberrydb/dev team for review and approval when your PR is ready🥳