Add migration notice to README (#38)

We are in the process of migrating this repo to our `core` monorepo.
Further changes to this repo won't be migrated over.

README.md

@@ -1,5 +1,7 @@
 # eth-json-rpc-provider
 
+<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>
+
 This package provides functions for creating an Ethereum provider from a [JSON-RPC engine](https://github.com/MetaMask/json-rpc-engine), or from middleware compatible with JSON-RPC engine.
 
 ## Installation