docs: add storybook local run instructions (#123)

superset-frontend/temporary_superset_ui/superset-ui/plugins/superset-ui-plugins/README.md

@@ -98,6 +98,17 @@ yarn build
 Each package defines its own build config, linting, and testing. You can have lerna run commands
 across all packages using the syntax `yarn run test` (or `yarn run test:watch` for watch mode) from the root `@superset-ui` directory.
 
+#### Storybook
+
+You can demo your changes by running the storybook demo locally with the following commands:
+
+```sh
+yarn install
+yarn build
+cd packages/superset-ui-plugins-demo
+yarn storybook
+```
+
 ### Committing
 
 This repository follows [conventional commits](https://www.conventionalcommits.org/en/v1.0.0-beta.3/) guideline for commit messages and has a `commitlint` hook which will require you to have the valid commit message before committing. You can use `npm run commit` to help you create a commit message.