Fix #166 - Add docs for validating default values (#168)

Co-authored-by: Hasan Ramezani <hasan.r67@gmail.com>

docs/index.md

@@ -104,6 +104,40 @@ print(Settings().model_dump())
 
     Check the [Environment variable names documentation](#environment-variable-names) for more information.
 
+## Validation of default values
+
+Unlike pydantic `BaseModel`, default values of `BaseSettings` fields are validated by default.
+You can disable this behaviour by setting `validate_default=False` either in `model_config`
+or on field level by `Field(validate_default=False)`:
+
+```py
+from pydantic import Field
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(validate_default=False)
+
+    # default won't be validated
+    foo: int = 'test'
+
+
+print(Settings())
+#> foo='test'
+
+
+class Settings1(BaseSettings):
+    # default won't be validated
+    foo: int = Field('test', validate_default=False)
+
+
+print(Settings1())
+#> foo='test'
+```
+
+Check the [Validation of default values](validators.md#validation-of-default-values) for more information.
+
 ## Environment variable names
 
 By default, the environment variable name is the same as the field name.