Fix broken README.md warning alert syntax

[ryanlua]

Fixes the broken markdown syntax for the "warning" alert in the README. Changes > **Warning** to > [!WARNING] so the alert renders correctly again.

Source

Update - 14 November 2023

• The initial syntax using e.g. **Note** isn't supported any longer.

Update - 21 July 2023

• A new syntax, [!NOTE], has been added, which will gradually replace the old one. However, the old syntax will continue to work for some time.

https://github.com/orgs/community/discussions/16925