Update documentation and provider-jet-template repo to cover creating a crossplane provider from a CUSTOM Terraform provider

[spkane]

What problem are you facing?

It appears like the documentation (and related Makefile) for generating a provider, only works with Terraform providers that are downloadable from a Terraform registry.

• https://github.com/crossplane/terrajet/blob/main/docs/generating-a-provider.md

If an organization has a custom/internal Terraform provider, or even one that is public like todo-for-terraform

• https://github.com/spkane/todo-for-terraform/releases/tag/v1.2.0
• https://github.com/spkane/todo-for-terraform/tree/master/terraform-provider-todo

it is not clear how to point at this, so that a crossplane provider can be created.

How could Terrajet help solve your problem?

Update the documentation and provider-jet-template to cover this (and similar) use cases.