docs: update docs to mention conditional fields

docs/models/index.md

@@ -6,14 +6,20 @@ Note: A model is called `DocType` in Frappe.js
 
 ### Fields
 
-Every model must have a set of fields (these become database columns). All fields must have
+Every model must have a set of fields (these become database columns). All fields may have the following properties:
 
-- `fieldname`: Column name in database / property name
+- `fieldname`: Column name in database / property name.
-- `fieldtype`: Data type ([see details](fields.md))
+- `fieldtype`: Data type ([see details](fields.md)).
-- `label`: Display label
+- `label`: Display label.
-- `required`: Is mandatory
+- `required`: Is mandatory.
-- `hidden`: Is hidden
+- `hidden`: Is hidden.
-- `disabled`: Is disabled
+- `disabled`: Is disabled.
+
+### Conditional Fields
+
+The following fields: `hidden`, `required` can be conditional, depending on the values of other fields.
+This is done by setting it's value to a function that receives an object with all the models fields and their values and returns a boolean.
+See _"Posting Date"_ in the example below.
 
 ### Example
 
@@ -28,7 +34,7 @@ module.exports = {
 			"fieldname": "subject",
 			"label": "Subject",
 			"fieldtype": "Data",
-			"reqd": 1
+			"required": 1
 		},
 		{
 			"fieldname": "description",
@@ -44,8 +50,14 @@ module.exports = {
 				"Closed"
 			],
 			"default": "Open",
-			"reqd": 1
+			"required": 1
-		}
+		},
+		{
+			"fieldname": "postingDate",
+			"label": "Posting Date",
+			"fieldtype": "Date",
+			"required": (doc) => doc.status === "Closed"
+		},
 	]
 }
 ```