docs(ui): add usage in README (#170)

packages/ui/README.md

@@ -4,6 +4,25 @@ A stylized UI component library built with [Reka UI](https://reka-ui.com/).
 
 To preview the components, refer to the [`stage-ui`](../stage-ui) package for instructions for running the Histoire UI storyboard.
 
+## Usage
+
+```shell
+ni @proj-airi/ui -D # from @antfu/ni, can be installed via `npm i -g @antfu/ni`
+pnpm i @proj-airi/ui -D
+yarn i @proj-airi/ui -D
+npm i @proj-airi/ui -D
+```
+
+```vue
+<script setup lang="ts">
+import { Button } from '@proj-airi/ui'
+</script>
+
+<template>
+  <Button>Click me</Button>
+</template>
+```
+
 ## Components
 
 * [Animations](src/components/Animations)