AMD: Note on AMD best practices

Fixes #14687
Closes gh-1478

AUTHORS.txt

@@ -215,3 +215,4 @@ John Paul <john@johnkpaul.com>
 Jakob Stoeck <jakob@pokermania.de>
 Christopher Jones <chris@cjqed.com>
 Forbes Lindesay <forbes@lindesay.co.uk>
+S. Andrew Sheppard <andrew@wq.io>

src/exports/amd.js

@@ -9,6 +9,12 @@ define([
 // derived from file names, and jQuery is normally delivered in a lowercase
 // file name. Do this after creating the global so that if an AMD module wants
 // to call noConflict to hide this version of jQuery, it will work.
+
+// Note that for maximum portability, libraries that are not jQuery should
+// declare themselves as anonymous modules, and avoid setting a global if an
+// AMD loader is present. jQuery is a special case. For more information, see
+// https://github.com/jrburke/requirejs/wiki/Updating-existing-libraries#wiki-anon
+
 if ( typeof define === "function" && define.amd ) {
 	define( "jquery", [], function() {
 		return jQuery;