Added documentation for optional properties.

website/src/docs/hotchocolate/defining-a-schema/input-object-types.md

@@ -195,6 +195,33 @@ We can also use record types, if we're on C# 9.0+. The equivalent to the above w
 public record BookingInput(string Title, string Author);
 ```
 
+## Optional Properties
+
+If we want our input type classes to contain optional properties, we can use the `Optional<T>` type or mark the properties of the class as `nullable`. It is important to also define a default value for any non-nullable property that is using the `Optional<T>` type by adding the `[DefaultValue]` attribute, otherwise the field will still be required when defining the input.
+
+```csharp
+public class BookInput
+{
+    [DefaultValue("")]
+    public Optional<string> Title { get; set; }
+    public string Author { get; set; }
+
+    public BookInput(string title, string author)
+    {
+        Title = title;
+        Author = author;
+    }
+}
+
+```
+
+Also with record types, the equivalent of the above would be:
+
+```csharp
+public record BookInput([property:DefaultValue("")]Optional<string> Title, string Author);
+
+```
+
 ## Oneof Input Objects
 
 Oneof Input Objects are a special variant of Input Objects where the type system asserts that exactly one of the fields must be set and non-null, all others being omitted. This is represented in introspection with the \_\_Type.oneField: Boolean field, and in SDL via the @oneOf directive on the input object.