docs: Add migration notice (#428)

We're migrating this to `/core` tomorrow.

Author: rekmarks

README.md

@@ -1,5 +1,7 @@
 # eth-json-rpc-middleware
 
+<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>
+
 Ethereum-related middleware for [`json-rpc-engine`](https://github.com/MetaMask/json-rpc-engine).
 
 See tests for usage details.