Add docs on how to release a new major protocol version. (#29552)

Releasing a new major protocol version requires coordination between a
few different projects and codebases, and it's easy to forget a step.
This commit introduces a doc to keep track of these steps, making it
less likely one will be omitted or forgotten.

docs/plugin-protocol/releasing-new-version.md

@@ -0,0 +1,53 @@
+# Releasing a New Version of the Protocol
+
+Terraform's plugin protocol is the contract between Terraform's plugins and
+Terraform, and as such releasing a new version requires some coordination
+between those pieces. This document is intended to be a checklist to consult
+when adding a new major version of the protocol (X in X.Y) to ensure that
+everything that needs to be is aware of it.
+
+## New Protobuf File
+
+The protocol is defined in protobuf files that live in the hashicorp/terraform
+repository. Adding a new version of the protocol involves creating a new
+`.proto` file in that directory. It is recommended that you copy the latest
+protocol file, and modify it accordingly.
+
+## New terraform-plugin-go Package
+
+The
+[hashicorp/terraform-plugin-go](https://github.com/hashicorp/terraform-plugin-go)
+repository serves as the foundation for Terraform's plugin ecosystem. It needs
+to know about the new major protocol version. Either open an issue in that repo
+to have the Plugin SDK team add the new package, or if you would like to
+contribute it yourself, open a PR. It is recommended that you copy the package
+for the latest protocol version and modify it accordingly.
+
+## Update the Registry's List of Allowed Versions
+
+The Terraform Registry validates the protocol versions a provider advertises
+support for when ingesting providers. Providers will not be able to advertise
+support for the new protocol version until it is added to that list.
+
+## Update Terraform's Version Constraints
+
+Terraform only downloads providers that speak protocol versions it is
+compatible with from the Registry during `terraform init`. When adding support
+for a new protocol, you need to tell Terraform it knows that protocol version.
+Modify the `SupportedPluginProtocols` variable in hashicorp/terraform's
+`internal/getproviders/registry_client.go` file to include the new protocol.
+
+## Test Running a Provider With the Test Framework
+
+Use the provider test framework to test a provider written with the new
+protocol. This end-to-end test ensures that providers written with the new
+protocol work correctly wtih the test framework, especially in communicating
+the protocol version between the test framework and Terraform.
+
+## Test Retrieving and Running a Provider From the Registry
+
+Publish a provider, either to the public registry or to the staging registry,
+and test running `terraform init` and `terraform apply`, along with exercising
+any of the new functionality the protocol version introduces. This end-to-end
+test ensures that all the pieces needing to be updated before practitioners can
+use providers built with the new protocol have been updated.