docs: add example

Author: joscha

README.md

@@ -1,4 +1,38 @@
 # pydantic-flatten-rootmodel
 
 Library to transform a [Pydantic](https://pydantic.dev/)
-[RootModel](https://docs.pydantic.dev/latest/api/root_model/) into a flattened BaseModel.
+[RootModel](https://docs.pydantic.dev/latest/api/root_model/)
+with discriminated unions into a flattened BaseModel.
+
+```py
+from pydantic-flatten-rootmodel import flatten_root_model
+
+class Cat(BaseModel):
+    pet_type: Annotated[Literal["cat"], Field()]
+    meow: str
+
+
+class Dog(BaseModel):
+    pet_type: Annotated[Literal["dog"], Field()]
+    bark: str
+
+class Pet(RootModel[Cat | Dog]):
+    root: Annotated[Cat | Dog, Field(discriminator="pet_type")]
+
+
+FlattenedPet = flatten_root_model(Pet)
+```
+
+ would result in `FlattenedPet` to have this shape:
+
+ ```py
+ class FlattenedPet(BaseModel):
+    pet_type: Annotated[Union[Literal["cat"], Literal["dog"]]]
+    bark: Union[str, None]
+    meow: Union[str, None]
+ ```
+
+ This can for example be leveraged by [dlt](https://dlthub.com) for it's
+ [schema definition](https://dlthub.com/docs/general-usage/resource#define-a-schema-with-pydantic).
+ Without flattening it, the discriminated union is not recognized correctly
+ when setting up the table schema.