Mention terraform-plugin-framework in README (#991)

Author: bflad

README.md

@@ -2,7 +2,9 @@
 
 # Terraform Plugin SDK
 
-This SDK enables building Terraform plugin which allows Terraform's users to manage existing and popular service providers as well as custom in-house solutions.
+This SDK enables building Terraform plugin which allows Terraform's users to manage existing and popular service providers as well as custom in-house solutions. The SDK is stable and broadly used across the provider ecosystem.
+
+For new provider development it is recommended to investigate [`terraform-plugin-framework`](https://github.com/hashicorp/terraform-plugin-framework), which is a reimagined provider SDK that supports additional capabilities. Refer to the [Which SDK Should I Use?](https://terraform.io/docs/plugin/which-sdk.html) documentation for more information about differences between SDKs.
 
 Terraform itself is a tool for building, changing, and versioning infrastructure safely and efficiently. You can find more about Terraform on its [website](https://www.terraform.io) and [its GitHub repository](https://github.com/hashicorp/terraform).