Add Festivo Public Holiday

Signed-off-by: CipherGirl <ciphergirlmeow@gmail.com>

README.md

@@ -234,6 +234,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Calendar Index](https://www.calendarindex.com/) | Worldwide Holidays and Working Days | `apiKey` | Yes | Yes |
 | [Church Calendar](http://calapi.inadiutorium.cz/) | Catholic liturgical calendar | No | No | Unknown |
 | [Czech Namedays Calendar](https://svatky.adresa.info) | Lookup for a name and returns nameday date | No | No | Unknown |
+| [Festivo Public Holidays](https://docs.getfestivo.com/docs/products/public-holidays-api/intro) | Fastest and most advanced public holiday and observance service on the market | `apiKey` | Yes | Yes |
 | [Google Calendar](https://developers.google.com/google-apps/calendar/) | Display, create and modify Google calendar events | `OAuth` | Yes | Unknown |
 | [Hebrew Calendar](https://www.hebcal.com/home/developer-apis) | Convert between Gregorian and Hebrew, fetch Shabbat and Holiday times, etc | No | No | Unknown |
 | [Holidays](https://holidayapi.com/) | Historical data regarding holidays | `apiKey` | Yes | Unknown |