Consider adding contributing guidelines

[sreetamdas]

Hey! 👋

I've contributed to the repo in the past (#376 and #478), and on both instances I've looked around a CONTRIBUTING.md with guidelines and/or instructions around how to setup the repo locally for development. Could we please add a CONTRIBUTING.md to help people who might be looking to help and contribute to this project? Thanks!

Additionally, this project uses certain @corex/* packages—unfortunately, there is no documentation around these since the link on the npm page is broken. One can still understand what the packages are doing by inferring it from their usage, but that involves guesswork. 😅