Adding CalendarIndex Holidays API (#767)

* Adding CalendarIndex Holidays API

* Removing . from the end of the description

* Remove "API" from Description

README.md

@@ -129,6 +129,7 @@ API | Description | Auth | HTTPS | CORS |
 ### Calendar
 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](http://svatky.adresa.info/) | Lookup for a name and returns nameday date | No | No | Unknown |
 | [Google Calendar](https://developers.google.com/google-apps/calendar/) | Display, create and modify Google calendar events | `OAuth` | Yes | Unknown |