add @hasDefault to smart-tags.md (#346)

Author: TimoStolz

src/pages/postgraphile/smart-tags.md

@@ -142,6 +142,16 @@ comment on column my_schema.my_table.my_column is
   E'@deprecated Use myOtherColumn instead.';
 ```
 
+### @hasDefault
+
+You can indicate that a column has a default - be that implemented via a trigger,
+or maybe as an instead of insert rule - so the field will be nullable in create*
+mutations.
+
+```sql
+comment on column my_schema.my_table.my_column is E'@hasDefault';
+```
+
 #### @name
 
 You can add a smart tag to an entity to rename that entity. For tables, columns,