How to add petal.build to a Phoenix Project

@SimonLab you recently added https://petal.build/components to the MVP 👌
Please document the steps here so that other people can learn/replicate in their project.

# Todo

+ [ ] Add `petal.build` section below [`Tailwind` in `Phoenix`](https://github.com/dwyl/learn-tailwind/tree/b11b902873e008ecd64f56070fa6826d5d8a2c69#more-detailed-example) in the `README.md` brief description of what `petal.build` is and
+ [ ] instructions for how to add it to a new `Phoenix` project

Thanks! 🙏