docs: add contributing.md

[bjohansebas]

This is part of adding the documentation that can be inherited from this repository. It is the same content as in https://github.com/expressjs/express/blob/master/Contributing.md, excluding expressjs/express#6087 and expressjs/express#6033.

[bjohansebas]

cc: @expressjs/express-tc

[bjohansebas]

Maybe we want the main contributing to be in admin and follow the style in Node.js.

Node.js admin: https://github.com/nodejs/admin/blob/main/CODE_OF_CONDUCT.md
Node.js .github: https://github.com/nodejs/.github/blob/main/CODE_OF_CONDUCT.md

[wesleytodd]

I dont like that added complexity of the admin repo. I was thinking we should make the .github repo the source for all we can. Honestly the name of the repo doesn't really matter as long as the docs are in the right place and the links are all correct.

My main concern with this current form is that we dont have a good concept in here of the difference between contributing to express vs all the other packages. I think we should keep this in a branch and open PRs against it here until we are satisfied. This whole consolidation effort was one of the main things I wanted to do soon, so let me just leave this comment here and circle back in a week or so.

[ctcpip]

Agree with Wes. Frankly, we have a backlog of updating the contributing guide, as well as the security policy -- something we delayed doing until after the audit was complete and we were in 2025.