document how to start a Django project

Author: timgraham

README.md

@@ -48,6 +48,64 @@ DATABASES = {
 `OPTIONS` is an optional dictionary of parameters that will be passed to
 [`MongoClient`](https://pymongo.readthedocs.io/en/stable/api/pymongo/mongo_client.html).
 
+In your Django settings, you must specify that all models should use
+`MongoAutoField`.
+
+```python
+DEFAULT_AUTO_FIELD = "django_mongodb.fields.MongoAutoField"
+```
+
+This won't override any apps that have an `AppConfig` that specifies
+`default_auto_field`. For those apps, you'll need to create a custom
+`AppConfig`.
+
+For example, you might create `mysite/apps.py` like this:
+
+```python
+from django.contrib.admin.apps import AdminConfig
+from django.contrib.auth.apps import AuthConfig
+from django.contrib.contenttypes.apps import ContentTypesConfig
+
+
+class MongoAdminConfig(AdminConfig):
+    default_auto_field = "django_mongodb.fields.MongoAutoField"
+
+
+class MongoAuthConfig(AuthConfig):
+    default_auto_field = "django_mongodb.fields.MongoAutoField"
+
+
+class MongoContentTypesConfig(ContentTypesConfig):
+    default_auto_field = "django_mongodb.fields.MongoAutoField"
+```
+
+Then replace each app reference in the `INSTALLED_APPS` setting with the new
+``AppConfig``. For example, replace  `'django.contrib.admin'` with
+`'mysite.apps.MongoAdminConfig'`.
+
+Because all models must use `MongoAutoField`, each third-party and contrib app
+you use needs to have its own migrations specific to MongoDB.
+
+For example, you might configure your settings like this:
+
+```python
+MIGRATION_MODULES = {
+    "admin": "mongo_migrations.admin",
+    "auth": "mongo_migrations.auth",
+    "contenttypes": "mongo_migrations.contenttypes",
+}
+```
+
+After creating a `mongo_migrations` directory, you can then run:
+
+```console
+$ python manage.py makemigrations admin auth contenttypes
+Migrations for 'admin':
+  mongo_migrations/admin/0001_initial.py
+    - Create model LogEntry
+...
+```
+
 ## Known issues and limitations
 
 - The following `QuerySet` methods aren't supported: