Update documentation

Author: andrew-farries

docs/README.md

@@ -1101,7 +1101,7 @@ Example **create table** migrations:
 
 A create constraint operation adds a new constraint to an existing table.
 
-Only `UNIQUE` and `CHECK` constraints are supported.
+`UNIQUE`, `CHECK` and `FOREIGN KEY` constraints are supported.
 
 Required fields: `name`, `table`, `type`, `up`, `down`.
 
@@ -1113,7 +1113,14 @@ Required fields: `name`, `table`, `type`, `up`, `down`.
     "table": "name of table",
     "name": "my_unique_constraint",
     "columns": ["col1", "col2"],
-    "type": "unique"
+    "type": "unique"| "check" | "foreign_key",
+    "check": "SQL expression for CHECK constraint",
+    "references": {
+      "name": "name of foreign key reference",
+      "table": "name of referenced table",
+      "columns": "[names of referenced columns]",
+      "on_delete": "ON DELETE behaviour, can be CASCADE, SET NULL, RESTRICT, or NO ACTION. Default is NO ACTION",
+    },
     "up": {
       "col1": "col1 || random()",
       "col2": "col2 || random()"
@@ -1130,7 +1137,7 @@ Example **create constraint** migrations:
 
 * [44_add_table_unique_constraint.json](../examples/44_add_table_unique_constraint.json)
 * [45_add_table_check_constraint.json](../examples/45_add_table_check_constraint.json)
-
+* [46_add_table_foreign_key_constraint.json](../examples/46_add_table_foreign_key_constraint.json)
 
 ### Drop column