Document steal.done

This adds docs for steal.done, fixes #489

docs/steal-done.md

@@ -0,0 +1,23 @@
+@property {function} steal.done
+@parent StealJS.functions
+
+A promise for when the application has loaded. Useful if you want to automatically run some code (like tests) after everything has loaded.
+
+@signature `steal.done()`
+
+@return {Promise} A promise that will resolve when the [System.configMain] and [System.main] have loaded.
+
+@body
+
+## Use
+
+Calling **steal.done** is useful in scenarios where you need to start some process after the dependency graph of your application has been fully imported. For example (used with [@steal]):
+
+    var steal = require("@steal");
+	var QUnit = require("qunit");
+
+	steal.done().then(function(){
+      QUnit.load();
+	});
+
+This will start running QUnit tests after all modules have loaded.