Add preview message on module home page

[dingmeng-xue]

Description

Add preview message on Databricks home page

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)
  • the markdown help files have been regenerated using the commands listed here

[adxsdkps]

Can one of the admins verify this patch?

[mikefrobbins]

What does it need to be installed separately from? I used the following message in the quickstart for Databricks that I wrote:

"While the Az.Databricks PowerShell module is in preview, you must install it separately from the Az PowerShell module using the following command: Install-Module -Name Az.Databricks -AllowPrerelease. Once the Az.Databricks PowerShell module is generally available, it becomes part of future Az PowerShell module releases and available natively from within Azure Cloud Shell."

[dingmeng-xue]

I just copy/past your content, except -AllowPrerelease is not required according to my test.

[dcaro]

LGTM

[mikefrobbins]

LGTM