Broken Contributing Guide link in README.md

Hello,

I found a minor issue in the `README.md`. The  `Contributing Guide` link is broken. If it's alright with the maintainers of this repo, I'd like to raise a PR to fix this.

`Contributing Guide` is pointing to `https://github.com/expressjs/express/blob/master/Contributing.md`. It should point to `https://github.com/expressjs/codemod/blob/main/CONTRIBUTING.md` 

Thanks