note on AMD best practices

Not surprisingly, jQuery has become the go-to example many developers looking to add an AMD definition to their library.  As we know, jQuery's near ubiquity means that certain AMD recommendations are not practical *for jQuery*.  Unfortunately, the jQuery the named module registration is referenced with the (reasonable) assumption that it's representative of AMD best practices.  For example discussions, see https://github.com/Leaflet/Leaflet/issues/1364. https://github.com/Leaflet/Leaflet/pull/1778, https://github.com/janl/mustache.js/pull/333, https://github.com/janl/mustache.js/pull/345 and https://github.com/mbostock/d3/pull/1672.

I'm hoping an extra comment will make it clear that jQuery is a special case.