瀑布流加载功能实现 && 文档补全

packages/waterfall/src/main.js

@@ -0,0 +1,14 @@
+import Waterfall from './directive.js';
+import Vue from 'vue';
+
+const install = function(Vue) {
+  Vue.directive('WaterfallLower', Waterfall('lower'));
+  Vue.directive('WaterfallUpper', Waterfall('upper'));
+};
+
+if (!Vue.prototype.$isServer) {
+  Vue.use(install);
+}
+
+Waterfall.install = install;
+export default Waterfall;