Inability to add the product to the cart/checkout - extend the documentation

[adamczykjac]

For all of those cracking their head around why the Gatsby Shopify Starter demo or their own direct deployment of this repo doesn't allow successfully adding any product to the cart/checkout.

Here's the solution I found in Shopify Forum:

You need to published the product to the sales channel of your private app creating in Shopify Admin (screenshot).

In my case it was a private app called "Gatsby Tryout".

I will add a PR to include that in the documentation.