Establish criteria and process to review middleware listed on the site

http://expressjs.com/en/resources/middleware.html#additional-middleware-modules lists some middleware modules that are note maintained by the Expressjs team.

We get requests to add more modules to this list (see #968 and #981 for example), but as @wesleytodd noted:

> Adding to the website is an explicit recommendation to the community, and I think we should be judicious about that.

So, I suggest we
1. Establish some basic criteria for listing middleware in the above page.
1. Review the existing list to ensure that all the modules meet those criteria.
1. Add a brief explanation of the criteria and process to http://expressjs.com/en/resources/contributing.html (or elsewhere, as appropriate).

Does anyone have suggestions for basic criteria for num. 1? @expressjs/expressjs-com