Added ERC20 example readme

example/erc20/README.md

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+## Mintable ERC20 implementation in Huff
+
+This is an ERC20 contract implemented in Huff to demonstrate the language's (very basic) syntax. It consists of three files:
+
+| filename             | description                                                                                                     |
+| -------------------- | --------------------------------------------------------------------------------------------------------------- |
+| `erc20.huff`         | Huff source file                                                                                                |
+| `erc20_interface.js` | JavaScript methods allowing the user to easily run Huff macros corresponding to ERC20 methods (e.g. `transfer`) |
+| `erc20.spec.js`      | Unit tests -- the Huff code really works!                                                                       |
+
+(Example unit tests can be run from `/huff/` with `yarn exampletest`.)
+
+The process by which the Huff code was created is documented in a series of blog posts on Medium:
+1. [About Huff, constructor, and function selector](https://medium.com/aztec-protocol/from-zero-to-nowhere-smart-contract-programming-in-huff-1-2-ba2b6de7fa83)
+2. [Events, mappings, and `transfer`](https://medium.com/aztec-protocol/from-zero-to-nowhere-smart-contract-programming-in-huff-2-3-5438ef7e5beb)
+3. [`totalSupply`, `balanceOf`, and `mint`](https://medium.com/aztec-protocol/from-zero-to-nowhere-smart-contract-programming-in-huff-3-4-6b347e23d66e)
+4. [Nested mappings, `allowance`, `approve`, and `transferFrom`](https://medium.com/aztec-protocol/from-zero-to-nowhere-smart-contract-programming-in-huff-4-4-9e6c34648992)