chore(payments): add mention of local storybook to payments README

packages/fxa-payments-server/README.md

@@ -8,6 +8,8 @@ This project uses [Storybook](https://storybook.js.org/) to show each screen wit
 
 You can view the Storybook built from the most recent master at http://mozilla.github.io/fxa/fxa-payments-server/
 
+In local development, `npm run storybook` should start a Storybook server at <http://localhost:6006> with hot module replacement to reflect live changes.
+
 ## Installation notes
 
 On Mac OS, `npm run test` may trigger an `EMFILE` error. In this case, to get tests running, you may need to `brew install watchman`. (If the watchman postinstall step fails, follow the instructions [here](https://stackoverflow.com/a/41320226) to change `/usr/local` ownership from root to your user account.)