Add AzPreview link to main readme

[isra-fel]

Description

Follow up for #15451
Preview: https://github.com/Azure/azure-powershell/blob/30ed45d9a1a7a2a3ed25c0ec7a2e169c7dcb5dde/README.md

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[isra-fel]

@dingmeng-xue pleae help review this PR. THanks