Update help message to include Yarn instructions

[askadityapandey]

This PR addresses the issue reported in phaser/phaser#6847 regarding missing Yarn instructions in the create-app tool.

Changes made:

• Updated the help message to include instructions for both npm and Yarn users
• Added examples for Yarn usage alongside npm examples

This should resolve the discrepancy between the instructions provided and the package manager used.

Fixes phaser/phaser#6847

[photonstorm]

Thanks for this PR, but please do not edit build files because the moment we generate a new build, all these changes will be lost. PRs should be made against the src files only.