Update README.md

What does this MR do?

Updating the landing page to https://about.gitlab.com/contributing/ (vs. CONTRIBUTING.md) that also have information on documentation, translation, and now UX. This is a more welcoming page for new contributors....

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides