shopify-app-express: document environment variable defaults

[sergiocasao]

WHY are these changes introduced?

@shopify/shopify-app-express automatically reads the following environment variables when they are not explicitly passed to shopifyApp():

• SHOPIFY_API_KEY
• SHOPIFY_API_SECRET
• HOST (split into hostScheme and hostName)
• SCOPES

This behavior is implemented in apiConfigWithDefaults() in the source, but is not mentioned anywhere in the README or docs.

see #3110

WHAT is this pull request doing?

Add a section to the README documenting the supported environment variables

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used pnpm changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[sergiocasao]

I have signed the CLA!