Update README.md

Author: Zignature

system/user/addons/json/README.md

@@ -185,6 +185,27 @@ The data will be the Unix timestamp, accurate to milliseconds. This is because t
 your_date_field: 1385661660000
 ```
 
+##### File Grid
+
+The data will include an array of File Grid rows, including the row_id, the url to the file and the column names:
+
+```
+"your_grid_field": [
+	{
+		"row_id": 1,
+    "file": "the-url-to-your-file",
+		"my_col_name": "foo",
+		"other_col_name": "bar"
+	},
+	{
+		"row_id": 2,
+    "file": "the-url-to-your-file",
+		"my_col_name": "baz",
+		"other_col_name": "qux"
+	}
+]
+```
+
 ##### Fluid
 
 The data will include an array of custom fields ordered by Fluid field order.