feat: added Country rule documentation

Author: andrepimpao

docs/03x-rules-country.md

@@ -0,0 +1,55 @@
+# Country
+
+Validates that a value is a valid country code.
+
+```php
+Country(
+    string $code = 'alpha-2',
+    string $message = 'The "{{ name }}" value is not a valid country code, "{{ value }}" given.'
+);
+```
+
+## Basic Usage
+
+```php
+// Default alpha-2 code
+Validator::country()->validate('PT'); // true
+Validator::country(code: 'alpha-2')->validate('PT'); // true
+
+// Alpha-3 code
+Validator::country(code: 'alpha-3')->validate('PRT'); // true
+```
+
+> **Note**
+> An `UnexpectedValueException` will be thrown when the `code` value is not a valid option.
+
+> **Note**
+> An `UnexpectedValueException` will be thrown when the input value is not a `string`.
+
+## Options
+
+### `code`
+
+type: `string` default: `alpha-2`
+
+Set code type to validate the country. 
+Check the [official country codes](https://en.wikipedia.org/wiki/ISO_3166-1#Current_codes) list for more information.
+
+Available options:
+
+- `alpha-2`: two-letter code
+- `alpha-3`: three-letter code
+
+### `message`
+
+type `string` default: `The "{{ name }}" value is not a valid country code, "{{ value }}" given.`
+
+Message that will be shown if the input value is not a valid country code.
+
+The following parameters are available:
+
+| Parameter     | Description               |
+|---------------|---------------------------|
+| `{{ value }}` | The current invalid value |
+| `{{ name }}`  | Name of the invalid value |
+| `{{ code }}`  | Selected code type        |