chore: add README for better-auth adapter (#425)

* chore: add README for better-auth adapter

* typo

Author: ymc9

packages/auth-adapters/better-auth/README.md

@@ -0,0 +1,33 @@
+# ZenStack Better-Auth Adapter
+
+This package provides a database adapter for [better-auth](https://better-auth.com). It allows you to use ZenStack ORM as the database backend for better-auth.
+
+## Installation
+
+```bash
+npm install @zenstackhq/better-auth@next
+```
+
+## Configuration
+
+```ts
+import { zenstackAdapter } from '@zenstackhq/better-auth';
+
+// ZenStack ORM client
+import { db } from './db';
+
+const auth = new BetterAuth({
+    database: zenstackAdapter(db, {
+        provider: 'postgresql', // or 'sqlite'
+    }),
+    // other better-auth options...
+});
+```
+
+## Schema generation
+
+You can use the `@better-auth/cli` to populate better-auth's data models into your ZModel schema.
+
+```bash
+npx @better-auth/cli generate
+```