Add governance info to website and clean up CONTRIBUTING.md

[crandmck]

Some of the info in this doc duplicates stuff in README.md. This should also link to https://github.com/expressjs/express/blob/master/Contributing.md and other relevant docs.

We should also note that since it is under the expressjs org and part of Node Founation, the general governance policies are the same as for th expressjs/express repo.

I'd also like to add some notes about opening PRs and other conventions, perhaps noting that we have slightly less stringent policies than expressjs/express (i.e. we don't usually required you to squash commits, using the Big Green Merge button is generally OK, etc.)

[crandmck]

Let's go ahead and provide the information on contributing and governance on the website itself (not just links to docs in the repo), possibly with pointers to additional info.

Copied from expressjs/express#2902:

We need ... a central location to provide the governance documentation and resources for all the projects that are under the Express TC without copying files into every single repository. We need something like https://nodejs.org/en/foundation/ and other sections of the Node.js website

[crandmck]

This is done. If we need to make further changes or additions, let's open new issues.