Added Zipcode API from zipcodeapi.com (#831)

* Added Zipcode API from zipcodeapi.com

* Added backtick for authentication field

README.md

@@ -424,6 +424,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Uebermaps](https://uebermaps.com/api/v2) | Discover and share maps with friends | `apiKey` | Yes | Unknown |
 | [Utah AGRC](https://api.mapserv.utah.gov) | Utah Web API for geocoding Utah addresses | `apiKey` | Yes | Unknown |
 | [ViaCep](https://viacep.com.br) | Brazil RESTful zip codes API | No | Yes | Unknown |
+| [Zipcodeapi](https://www.zipcodeapi.com) | Find out possible zip codes for a city, distance between zip codes etc | `apiKey` | Yes | Unknown |
 | [Zippopotam](http://www.zippopotam.us) | Get information about place such as country, city, state, etc | No | No | Unknown |
 
 ### Government