feat: filter out countries without public holidays

Fairly minor update, no code change, just added a new test to cover the
flag that was recently added to the API and expanded the documentation
to also include it.

README.md

@@ -67,6 +67,14 @@ try {
 $holiday_api->countries();
 ```
 
+#### Fetch only countries with public holidays
+
+```php
+$holiday_api->countries([
+  'public' => true,
+]);
+```
+
 #### Fetch a supported country by code
 
 ```php