[Docs] Missing documentation about usage outside Laravel in Google Play Billing.

[imdhemy]

Discussed in #387

^{Originally posted by ShreeSoftech-Rupesh February 21, 2024}
I attempted to install your package in my CodeIgniter project, but I encountered an issue: the setup instructions for Google Play Billing are missing.

[mattli001]

Recently, I have been using packages to integrate Google Play Console's IAP. Here are two detailed configuration documents for reference:

1. Google Play Service Account Key (last updated 2024-05)
2. Android In-App Products Setup (last updated 2024-01)

Especially when encountering error

{
 "error": {
  "code": 401,
  "message": "The current user has insufficient permissions to perform the requested o (truncated...)"
}

it was by referring to the two documents that the problem was solved.