Add erc-20 to available extensions (#18)

Author: damianmarti

README.md

@@ -16,6 +16,7 @@ npx create-eth@latest -e <extension-name>
 - [eip-712](https://github.com/scaffold-eth/create-eth-extensions/tree/eip-712): An implementation of EIP-712, allowing you to send, sign, and verify typed messages in a user-friendly manner.
 - [ponder](https://github.com/scaffold-eth/create-eth-extensions/tree/ponder): This Scaffold-ETH 2 extension comes pre-configured with [ponder.sh](https://ponder.sh), providing an example to help you get started quickly.
 - [onchainkit](https://github.com/scaffold-eth/create-eth-extensions/tree/onchainkit): This Scaffold-ETH 2 extension comes pre-configured with [onchainkit](https://onchainkit.xyz/), providing an example to help you get started quickly.
+- [erc-20](https://github.com/scaffold-eth/create-eth-extensions/tree/erc-20): This extension introduces an ERC-20 token contract and demonstrates how to interact with it, including getting a holder balance and transferring tokens.
 
 ## Create your own extension