Update README.md with phpstan minimum file

[M-arcus]

This pull request includes several updates to the README.md file to improve clarity and accuracy in the documentation for reusable GitHub Actions and Workflows for Shopware extensions. The most important changes are:

Documentation improvements:

• Corrected the capitalization of "extensions" in the title to maintain consistency.
• Clarified the description of the phpstan job to specify that it installs Shopware 6, PHPStan, and the extension before running PHPStan.
• Added instructions for creating a phpstan.neon.dist file required for the PHPStan check, including specifying the parameters and paths.
• Updated the Build Zip section to use consistent capitalization for "extension zip" and "shopware-cli".
• Updated the Store Upload section to use consistent capitalization and clarify the recommendation to use the workflow_dispatch event for manual triggering.