Added brief docs for PROMETHEUS_EXPORT_MIGRATIONS setting per #49 (#87)

Author: devmonkey22

README.md

@@ -122,6 +122,10 @@ are exported, `django_migrations_applied_by_connection` and
 `django_migrations_unapplied_by_connection`. You may want to alert if
 there are unapplied migrations.
 
+If you want to disable the Django migration metrics, set the
+`PROMETHEUS_EXPORT_MIGRATIONS` setting to False.
+
+
 ### Monitoring and aggregating the metrics
 
 Prometheus is quite easy to set up. An example prometheus.conf to