Docs: Add section "Upgrading to EE" to README

Follow-up from: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7266

New doc or update?

• New documentation
• Update existing documentation

The problem

As a CE user I'd like to learn how to "Upgrade to Enterprise Edition" from the README.

We could add a section to the README pointing the user to the Upgrade to Enterprise Edition page.

Checklists

• Add the correct labels
• Add the correct milestone
• Ping technical writer