Update readme with simpler instructions

Instead of recommending shallow checkouts in our readme, just check out the whole repo. Similar to our [`Generative Art` blog post](https://blog.chain.link/how-to-create-generative-art-nfts/)

```
git clone https://github.com/smartcontractkit/smart-contract-examples.git
cd smart-contract-examples/ultimate-nft-repo

yarn
```