Add docs on defining custom python type for column type inference

docs/Inferring Custom Pythons Types.md

@@ -0,0 +1,45 @@
+`sqlalchemy-stubs` is able to infer the python type based on the column definition for SQLAlchemy models.
+Thus,
+
+```python
+class User(Base):
+    id = Column(Integer, primary_key=True)
+```
+
+allows `User().id` to have inferred type `int`. It can be useful to have custom Python types to strengthen type-checking guarantees,
+especially for fields such as uuids, emails, phone numbers, transaction tokens, and more. To achieve this, do the following:
+
+```python
+from sqlalchemy.dialects.postgresql import UUID  # import your desired column type
+from sqlalchemy.sql import sqltypes
+from typing import NewType, TYPE_CHECKING
+from uuid import uuid4 as gen_uuid
+
+Uuid = NewType('Uuid', str)  # define the custom Python type to use for typechecking
+
+
+
+# use this type checking idiom bcause TypeDecorator doesn't support __getitem__
+# see [this mypy doc](https://mypy.readthedocs.io/en/latest/common_issues.html#using-classes-that-are-generic-in-stubs-but-not-at-runtime) for full details.
+if TYPE_CHECKING:
+    BaseUuid = sqltypes.TypeDecorator[Uuid]
+else:
+    BaseUuid = sqltypes.TypeDecorator
+
+
+# define a new column field, giving it a type-decorator for your type, and inheriting from the column type. Order is important here.
+class PgUuid(BaseUuid):
+    impl = Uuid
+
+
+def uuid4():
+    return str(gen_uuid())
+
+
+# Now define a model using the column.
+class User(Base):
+    __tablename__ = 'users'
+    uuid = Column(PgUuid(), primary_key=True, default=uuid4)
+```
+
+With this class, `User().uuid` will now typecheck to `Uuid`.