fix: add Alembic integration docs for create_casbin_rule_model()

[Copilot]

Documents the create_casbin_rule_model() function added in #22 for managing CasbinRule table schema through Alembic migrations.

Changes

• Added "Alembic Integration" section showing how to use create_casbin_rule_model() with application's declarative base
• Included example of passing custom model to adapter via db_class parameter
• Fixed typo: AsyncAdaper → AsyncAdapter, AynscEnforcer → AsyncEnforcer

Usage

# Define model in your app using your declarative base
from casbin_async_sqlalchemy_adapter import create_casbin_rule_model
CasbinRule = create_casbin_rule_model(Base)

# Alembic can now auto-generate migrations
# alembic revision --autogenerate -m "Add casbin_rule table"

# Use custom model with adapter
adapter = casbin_async_sqlalchemy_adapter.Adapter(
    'sqlite+aiosqlite:///test.db',
    db_class=CasbinRule
)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[docs] add docs for the PR</issue_title>
<issue_description>Improve the README.md based on the newly merged PR: #22
You should write high-quality human-like docs for a modern OSS software. Don't write in the GPT-flavor. Don't write too long, keep it tidy docs. Don't use itemized list to describe things like "Use Cases", "Best practice" or "what it can do"
Don't add too long text, keep it short and to the point</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [docs] add docs for the PR #23

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.