Add migrated and archived notice to README (#182)

> This package has been migrated to our <a href="https://github.com/MetaMask/core"><code>core</code></a> monorepo, and this repository has been archived. Please note that all future development and feature releases will take place in the <a href="https://github.com/MetaMask/core"><code>core</code></a> repository.

- Partially implements #180

README.md

@@ -1,6 +1,6 @@
 # `@metamask/json-rpc-engine`
 
-<table><tr><td><p align="center"><b>⚠️ PLEASE READ ⚠️</b></p><p align="center">This package is currently being migrated to our <a href="https://github.com/MetaMask/core"><code>core</code></a> monorepo. Please do not make any commits to this repository while this migration is taking place, as they will not be transferred over.</p></td></tr></table>
+<table><tr><td><p align="center"><b>⚠️ PLEASE READ ⚠️</b></p><p align="center">This package has been migrated to our <a href="https://github.com/MetaMask/core"><code>core</code></a> monorepo, and this repository has been archived. Please note that all future development and feature releases will take place in the <a href="https://github.com/MetaMask/core"><code>core</code></a> repository.</p></td></tr></table>
 
 A tool for processing JSON-RPC requests and responses.