(wip) Add features doc

docs/features.md

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+# Django Subatomic's features
+
+## Intentional transaction management
+
+...
+
+## No queries wasted on unused savepoints
+
+...
+
+## Fast, production-like tests
+
+Django tests are usually wrapped in a database transaction
+which is rolled back when the test completes.
+This means that after-commit callbacks would usually never be run.
+
+To simulate realistic production behaviour,
+Subatomic runs after-commit callbacks when we exit the
+outermost [`transaction`][django_subatomic.db.transaction]
+(or [`transaction_if_not_already`][django_subatomic.db.transaction_if_not_already]) context.
+This emulates how the application will behave when deployed
+and means that tests reproduce realistic application behaviour
+without resorting to other more costly or error-prone testing strategies.
+
+## No more accidental immediate "after-commit" callbacks
+
+Subatomic's [`run_after_commit`][django_subatomic.db.run_after_commit] will ensure that
+after-commit callbacks are only registered when a transaction is open.
+If no transaction is open, an error will be raised.
+
+This is in contrast to the default behaviour of Django's [`on_commit`][on_commit] function,
+where after-commit callbacks outside of transactions are executed immediately.
+
+We choose to disallow immediate execution because it can be misleading and hide bugs.
+In particular, it can hide the fact that a transaction is missing or on a different database,
+which can make code read as though a callback will be deferred when it won't be.
+
+## Progressive integration support
+
+The larger your project is,
+the more likely you are to benefit from
+the guardrails offered by Django Subatomic's strict features.
+Paradoxically, the larger your project is,
+the more difficult it may be to enable them all.
+
+Because integrating Django Subatomic all at once
+could be a daunting task for larger projects,
+we provide some [settings to ease the integration process](/reference/settings).
+They allow you to roll out strict behaviour on a per-test basis
+using Django's `override_settings`.
+
+[on_commit]: https://docs.djangoproject.com/en/stable/topics/db/transactions/#django.db.transaction.on_commit
+[override_settings]: https://docs.djangoproject.com/en/stable/topics/testing/tools/#django.test.override_settings
+[TransactionTestCase]: https://docs.djangoproject.com/en/stable/topics/testing/tools/#transactiontestcase