Add documentation for the makemigrations override

docs/makemigrations.md

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+# makemigrations
+
+The linter can override behaviour of the [Django makemigrations command](https://docs.djangoproject.com/en/dev/ref/django-admin/#django-admin-makemigrations).
+
+Either:
+ * by specifying the `--lint` option in the command line
+ * by setting the `MIGRATION_LINTER_OVERRIDE_MAKEMIGRATIONS` Django setting to `True`
+
+## Example
+```
+$ python manage.py makemigrations --lint
+
+Migrations for 'app_correct':
+  tests/test_project/app_correct/migrations/0003_a_column.py
+    - Add field column to a
+Linting for 'app_correct':
+(app_correct, 0003_a_column)... ERR
+        NOT NULL constraint on columns
+Deleted tests/test_project/app_correct/migrations/0003_a_column.py
+```
+
+## Options
+
+Among the options that can be given, additionally to the default `makemigrations` options, you can count the options
+to configure what should be linted:
+* `--database DATABASE` - specify the database for which to generate the SQL. Defaults to *default*.
+* `--warnings-as-errors` - handle warnings as errors and therefore return an error status code if we should.
+* `--exclude-migration-tests MIGRATION_TEST_CODE [...]` - specify backward incompatible migration tests to be ignored using the code (e.g. ALTER_COLUMN).