feat: add usage instructions, example code to README (#6)

Author: siuhui

README.md

@@ -16,6 +16,60 @@ This is an Adapter for [PyCasbin](https://github.com/casbin/pycasbin) that imple
 pip install casbin_databases_adapter
 ```
 
+## Simple Example
+
+```python
+import casbin_databases_adapter
+import casbin
+from databases import Database
+import sqlalchemy
+from sqlalchemy import Table, Column, String, Integer
+from sqlalchemy.sql.ddl import CreateTable
+import asyncio
+
+DATABASE_URL = "sqlite+aiosqlite:///example.db"
+
+async def create_casbin_rule_table(db: Database):
+    metadata = sqlalchemy.MetaData()
+    table = Table(
+        "casbin_rules",
+        metadata,
+        Column("id", Integer, primary_key=True),
+        Column("ptype", String(255)),
+        Column("v0", String(255)),
+        Column("v1", String(255)),
+        Column("v2", String(255)),
+        Column("v3", String(255)),
+        Column("v4", String(255)),
+        Column("v5", String(255)),
+    )
+    q = CreateTable(table)
+    await db.execute(query=str(q))
+    return table
+
+async def main():
+    database = Database(DATABASE_URL)
+    await database.connect()
+    casbin_rule_table = await create_casbin_rule_table(database)
+    adapter = casbin_databases_adapter.DatabasesAdapter(db=database, table=casbin_rule_table)
+    
+    e = casbin.Enforcer('path/to/model.conf', adapter)
+    
+    sub = "alice"  # the user that wants to access a resource.
+    obj = "data1"  # the resource that is going to be accessed.
+    act = "read"  # the operation that the user performs on the resource.
+    
+    if e.enforce(sub, obj, act):
+        # permit alice to read data1
+        pass
+    else:
+        # deny the request, show an error
+        pass
+
+# run the main function
+asyncio.run(main())
+```
+
 ### Getting Help
 
 - [PyCasbin](https://github.com/casbin/pycasbin)