fixup: docs

README.md

@@ -264,6 +264,21 @@ when(overloaded).calledWith().thenReturn(null)
 when<() => null>(overloaded).calledWith().thenReturn(null)
 ```
 
+#### Fallback
+
+By default, if arguments do not match, a vitest-when stub will no-op and return `undefined`. You can customize this fallback by configuring your own unconditional behavior on the mock using Vitest's built-in [mock API][].
+
+```ts
+const spy = vi.fn().mockReturnValue('you messed up!')
+
+subject.when(spy).calledWith('hello').thenReturn('world')
+
+spy('hello') // "world"
+spy('jello') // "you messed up!"
+```
+
+[mock API]: https://vitest.dev/api/mock.html
+
 ### `.thenReturn(value: TReturn)`
 
 When the stubbing is satisfied, return `value`