Add v2 Upgrade Guide and Learn docs to the README

Add a link to the README for the v2 upgrade guide, and another to the great tutorial the learn.hashicorp.com site has.

We should probably holistically redesign this README at some point, but that feels like a problem for another day.

Author: Paddy

README.md

@@ -18,6 +18,10 @@ The Terraform Plugin SDK is built in Go, and uses the [support policy](https://g
 
 Currently, that means Go **1.14** or later must be used when building a provider with the SDK.
 
+## Getting Started
+
+See the [Call APIs with Terraform Providers](https://learn.hashicorp.com/collections/terraform/providers) guide on [learn.hashicorp.com](https://learn.hashicorp.com) for a guided tour of provider development.
+
 ## Documentation
 
 See [Extending Terraform](https://www.terraform.io/docs/extend/index.html) section on the website.
@@ -51,6 +55,10 @@ See [Extending Terraform](https://www.terraform.io/docs/extend/index.html) secti
 
 Migrating to the standalone SDK v1 is covered on the [Plugin SDK section](https://www.terraform.io/docs/extend/plugin-sdk.html) of the website.
 
+## Migrating to SDK v2 from SDK v1
+
+Migrating to the v2 release of the SDK is covered in the [v2 Upgrade Guide](https://www.terraform.io/docs/extend/guides/v2-upgrade-guide.html) of the website.
+
 ## Versioning
 
 The Terraform Plugin SDK is a [Go module](https://github.com/golang/go/wiki/Modules) versioned using [semantic versioning](https://semver.org/).