document async locale middleware

Author: amirreza8002

docs/middleware/locale_middleware.md

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+# AsyncLocaleMiddleware
+
+it works exactly like django's [LocaleMiddleware](https://docs.djangoproject.com/en/5.1/ref/middleware/#module-django.middleware.locale)
+except that it's fully async
+read django's [Internationalization](https://docs.djangoproject.com/en/5.1/topics/i18n/translation/) documentations for a more in depth look.
+
+### Usage:
+remove django's `django.middleware.locale.LocaleMiddleware` from the `MIDDLEWARE` setting and add
+`django_async_extensions.middleware.locale.AsyncLocaleMiddleware` in it's place.
+
+**note**: this middleware like other middlewares provided in this package can work alongside sync middlewares, and can handle sync views.