Add deprecation notice for hardhat-etherscan

Author: fvictorio

packages/hardhat-etherscan/README.md

@@ -0,0 +1,13 @@
+[![npm](https://img.shields.io/npm/v/@nomiclabs/hardhat-etherscan.svg)](https://www.npmjs.com/package/@nomiclabs/hardhat-etherscan) [![hardhat](https://hardhat.org/buidler-plugin-badge.svg?1)](https://hardhat.org)
+
+# hardhat-etherscan
+
+The `@nomiclabs/hardhat-etherscan` plugin is deprecated in favor of our new [`@nomicfoundation/hardhat-verify`](/packages/hardhat-verify) plugin.
+
+## Migrating to `hardhat-verify`
+
+`hardhat-verify` is a drop-in replacement of `hardhat-etherscan`. To migrate to it:
+
+1. Uninstall the `@nomiclabs/hardhat-etherscan` package
+2. Install the `@nomicfoundation/hardhat-verify` package
+3. Update your Hardhat config to import `@nomicfoundation/hardhat-verify` instead of `@nomiclabs/hardhat-etherscan`