Show sponsors in the README or on the website.

[bjohansebas]

This is part of the requirements to be able to use the platforms that could be used in the performance group (see expressjs/perf-wg#5 (comment)). I'm opening this issue to track it, since a decision needs to be made about where this should be displayed (@wesleytodd likely has more context on the terms).

I suppose @expressjs/docs-wg should be involved here as well, since this involves the website.