docs: Describe configuration options

Author: pl-github

README.md

@@ -37,3 +37,16 @@ $project = new Project(
 ```shell
 $ composer require plook/type-guard
 ```
+
+## Configuration
+
+### Setting the default target time zone of `DateTimeImmutable` objects
+```php
+Convert::timeZone('Australia/Adelaide');
+Convert::timeZone(new DateTimeZone('Australia/Adelaide'));
+```
+
+### Setting the default format of date time strings
+```php
+Convert::dateTimeFormat(DateTimeInterface::ATOM);
+```