document async security middleware

Author: amirreza8002

docs/middleware/security_middleware.md

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+# AsyncSecurityMiddleware
+
+it works exactly like django's [SecurityMiddleware](https://docs.djangoproject.com/en/5.1/ref/middleware/#module-django.middleware.security)
+except that it's fully async
+
+### Usage:
+remove django's `django.middleware.security.SecurityMiddleware` from the `MIDDLEWARE` setting and add
+`django_async_extensions.middleware.security.AsyncSecurityMiddleware` in it's place.
+
+**note**: this middleware like other middlewares provided in this package can work alongside sync middlewares, and can handle sync views.