`CONTRIBUTING.md` does not contain description how to add a notebook

Hi there,

I'd be interested in contributing some notebooks (eventually). However, `CONTRIBUTING.md` currently does not contain a description how to do it on a technical as well as process level (I guess how PRs are managed here?).

A link to [Katharine Hyatt's description](https://kshyatt.github.io/post/firstjuliapr/) might be sufficient. However, I'm assuming that you want to adapt it to this repo, specifically, since you probably want to have very novice contributors in mind as well.

Also: keep up the good work in general :-)