docs: add mono-repo migration notice

Author: cwillisf

README.md

@@ -1,4 +1,26 @@
 ## scratch-vm
+
+## **⚠️ NOTICE: Repository Migration to Mono-Repo ⚠️**
+
+The Scratch Team has migrated the `scratch-vm` module into a new mono-repo,
+[`scratch-editor`](https://github.com/scratchfoundation/scratch-editor). This independent `scratch-vm` repository
+**will be archived**. Any new issues or pull requests should be opened in the mono-repo.
+
+The new mono-repo version of `scratch-vm` is published to the NPM registry as
+[`@scratch/scratch-vm`](https://www.npmjs.com/package/@scratch/scratch-vm).
+
+**Contributors:**
+
+* I would like to thank all past contributors for their work on this repository.
+* If you are aware of valuable issues or pull requests, please consider re-opening them in the mono-repo. If you do
+  so, please link the new issue or pull request to the original one in this repository to help others find it and to
+  reduce the chance of duplicate work.
+* We apologize for the inconvenience and greatly appreciate your help with this transition!
+
+For more information, see the [`scratch-editor` repository on GitHub](https://github.com/scratchfoundation/scratch-editor).
+
+## Overview
+
 #### Scratch VM is a library for representing, running, and maintaining the state of computer programs written using [Scratch Blocks](https://github.com/scratchfoundation/scratch-blocks).
 
 [![CI/CD](https://github.com/scratchfoundation/scratch-vm/actions/workflows/ci-cd.yml/badge.svg)](https://github.com/scratchfoundation/scratch-vm/actions/workflows/ci-cd.yml)