Mention add_column_with_default in downtime guide

[ci skip]

doc/development/what_requires_downtime.md

@@ -31,6 +31,14 @@ operation, even when using `ALGORITHM=INPLACE` and `LOCK=NONE`. This means
 downtime _may_ be required when modifying large tables as otherwise the
 operation could potentially take hours to complete.
+Adding a column with a default value _can_ be done without requiring downtime
+when using the migration helper method
+`Gitlab::Database::MigrationHelpers#add_column_with_default`. This method works
+similar to `add_column` except it updates existing rows in batches without
+blocking access to the table being modified. See ["Adding Columns With Default
+Values"](migration_style_guide.html#adding-columns-with-default-values) for more
+information on how to use this method.
+
 ## Dropping Columns
 On PostgreSQL you can safely remove an existing column without the need for