docs: Add migration notice (#352)

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

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Author: rekmarks

README.md

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 # @metamask/eth-block-tracker
 
+<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 module walks the Ethereum blockchain, keeping track of the latest block. It uses a web3 provider as a data source and will continuously poll for the next block.
 
 ## Installation