Closes #3600 - Updates README.md to include Launchpad preview message

[justinrobots]

Description

Updates the README.md to include a message explaining when Launchpad will be out of preview, with a link to upgrade to Pro at a discounted rate. Closes #3600

Checklist

• I have followed the guidelines in the Contributing document
• My changes follow the coding style of this project
• My changes build without any errors or warnings
• My changes have been formatted and linted
• My changes include any required corresponding changes to the documentation (including CHANGELOG.md and README.md)
• My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses

[d13]

LGTM