[Shopify-Next] Review and update the readme

[Megio]

Some step in the setup paragraphs are wrong or not clear, review the readme file and update it.

• [ ] How to set up env vars (attach some screenshot)

• [ ] Explicitly underline what API calls have some hard-coded data (for example some collections name are hard coded)

• [ ] Describe how to set up footer and header menu data inside Shopify store