chore: add docs

Author: sxzz

README.md

@@ -2,9 +2,8 @@
 
 [![Unit Test](https://github.com/unplugin/unplugin-quansync/actions/workflows/unit-test.yml/badge.svg)](https://github.com/unplugin/unplugin-quansync/actions/workflows/unit-test.yml)
 
-undefined
-
-
+Write async functions, get both async and sync functions with
+[quansync](https://github.com/antfu/quansync) and compiler magics.
 
 ## Installation
 
@@ -98,6 +97,33 @@ export default {
 
 <br></details>
 
+## Usage
+
+```ts
+import fs from 'node:fs'
+import { quansyncMacro } from 'quansync'
+
+// Create an quansync function by providing `sync` and `async` implementations
+const readFile = quansyncMacro({
+  sync: (path: string) => fs.readFileSync(path),
+  async: (path: string) => fs.promises.readFile(path),
+})
+
+// Create an quansync function by providing a **async** function
+const myFunction = quansyncMacro(async function (filename) {
+  // Use `await` to call another quansync function
+  const code = await readFile(filename, 'utf8')
+
+  return `// some custom prefix\n${code}`
+})
+
+// Use it as a sync function
+const result = myFunction.sync('./some-file.js')
+
+// Use it as an async function
+const asyncResult = await myFunction.async('./some-file.js')
+```
+
 ## Sponsors
 
 <p align="center">

package.json

@@ -2,7 +2,7 @@
   "name": "unplugin-quansync",
   "version": "0.0.0",
   "packageManager": "[email protected]",
-  "description": "undefined",
+  "description": "Write async functions, get both async and sync functions",
   "type": "module",
   "keywords": [
     "unplugin",