@storybook/addon-notes: support global decorator

Support the global decorator usage supplied in [the README](https://github.com/storybooks/storybook/blob/9f30442098e5350f5ba4872887cf9d1a03146e74/addons/ondevice-notes/README.md#getting-started)

types/storybook__addon-notes/index.d.ts

@@ -17,5 +17,9 @@ export type WithNotesOptions = string | {
     markdownOptions?: MarkedOptions;
 };
 
+// It would be preferable to infer the argument types, but that requires TS v 3.1
+// export function withNotes(...args: StoryDecorator extends (...a: infer A) => any ? A : never): ReturnType<StoryDecorator>;
+export function withNotes(story: RenderFunction, context: { kind: string, story: string }): ReturnType<StoryDecorator>;
+// Less-preferred but still supported:
 export function withNotes(options?: WithNotesOptions): StoryDecorator;
 export function withMarkdownNotes(markdown: string, options?: MarkedOptions): StoryDecorator;

types/storybook__addon-notes/storybook__addon-notes-tests.tsx

@@ -1,8 +1,11 @@
 import * as React from 'react';
 
-import { storiesOf } from '@storybook/react';
+import { addDecorator, storiesOf } from '@storybook/react';
 import { withNotes, withMarkdownNotes, } from '@storybook/addon-notes';
 
+// New, preferred global registration:
+addDecorator(withNotes);
+
 const SIMPLE_MARKDOWN = `
 ## Markdown for component