geoPlugin IP geolocation (public-apis#817)

* adding geoplugin to the geocoding section

geoPlugin - the original and largest free IP geolocation web service API
geoplugin.com offers multiple geocoding APIs, including IP geolocation, currency conversion, reverse geocoding since 2004

We were the first free IP geolocation web service on the net and handle around 1/10 of the global traffic that google.com handles (live stats available on geoplugin.com)

All this for free (limit 120 requests a minute). 

No API key or registration required to use the API.

* Update README.md

Author: inputsprocket

README.md

@@ -391,6 +391,7 @@ API | Description | Auth | HTTPS | CORS |
 | [GeoApi](https://api.gouv.fr/api/geoapi.html) | French geographical data | No | Yes | Unknown |
 | [Geocode.xyz](https://geocode.xyz/) | Provides worldwide forward/reverse geocoding, batch geocoding and geoparsing | No | Yes | Unknown |
 | [GeoNames](http://www.geonames.org/export/web-services.html) | Place names and other geographical data | No | No | Unknown |
+| [geoPlugin](https://www.geoplugin.com) | IP geolocation and currency conversion | No | Yes | Yes |
 | [Google Earth Engine](https://developers.google.com/earth-engine/) | A cloud-based platform for planetary-scale environmental data analysis | `apiKey` | Yes | Unknown |
 | [Google Maps](https://developers.google.com/maps/) | Create/customize digital maps based on Google Maps data | `apiKey` | Yes | Unknown |
 | [GraphLoc](https://www.graphloc.com) | Free GraphQL IP Geolocation API | No | Yes | Unknown |