add description for dynamic fields

Author: mizdra

README.md

@@ -89,12 +89,13 @@ The factory generates strictly typed mock data.
 ```ts
 // simple
 const book0 = await BookFactory.build();
-expect(book0).toStrictEqual({
-  __typename: 'Book',
-  id: 'Book-0',
-  title: 'apple',
-  author: undefined,
-});
+console.log(book0);
+// {
+//   __typename: 'Book',
+//   id: 'Book-0',
+//   title: 'apple',
+//   author: undefined,
+// }
 assertType<{
   __typename: 'Book';
   id: string;
@@ -106,17 +107,18 @@ assertType<{
 const book1 = await BookFactory.build({
   author: await AuthorFactory.build(),
 });
-expect(book1).toStrictEqual({
-  __typename: 'Book',
-  id: 'Book-1',
-  title: 'orange',
-  author: {
-    __typename: 'Author',
-    id: 'Author-0',
-    name: 'Tom',
-    books: undefined,
-  },
-});
+console.log(book0);
+// {
+//   __typename: 'Book',
+//   id: 'Book-1',
+//   title: 'orange',
+//   author: {
+//     __typename: 'Author',
+//     id: 'Author-0',
+//     name: 'Tom',
+//     books: undefined,
+//   },
+// }
 assertType<{
   __typename: 'Book';
   id: string;
@@ -134,6 +136,23 @@ assertType<{
 
 The library has several notable features. And many of them are inspired by [FactoryBot](https://thoughtbot.github.io/factory_bot/).
 
+### Dynamic Fields
+
+The `dynamic` function allows you to define fields with a dynamic value.
+
+```ts
+const BookFactory = defineBookFactory({
+  defaultFields: {
+    id: dynamic(() => faker.datatype.uuid()),
+    title: 'Yuyushiki',
+  },
+});
+expect(await BookFactory.build()).toStrictEqual({
+  id: expect.any(String), // Randomly generated UUID
+  title: 'Yuyushiki',
+});
+```
+
 ### Sequences
 
 Sequences allow you to build sequentially numbered data.