Update README.md (#985)

Added bng2latlong API, which converts British easting and northing into latitude and longitude. Free to use for any purpose.

README.md

@@ -406,6 +406,7 @@ API | Description | Auth | HTTPS | CORS |
 | [adresse.data.gouv.fr](https://adresse.data.gouv.fr) | Address database of France, geocoding and reverse | No | Yes | Unknown |
 | [Battuta](https://battuta.medunes.net) | A (country/region/city) in-cascade location API | `apiKey` | Yes | Unknown |
 | [Bing Maps](https://www.microsoft.com/maps/) | Create/customize digital maps based on Bing Maps data | `apiKey` | Yes | Unknown |
+| [bng2latlong](https://www.getthedata.com/bng2latlong) | Convert a British OSGB36 easting and northing (British National Grid) to WGS84 latitude and longitude | No | Yes | Yes |
 | [City Context](https://www.citycontext.com/api-reference#/) | Crime, school and transportation data for US cities | `apiKey` | Yes | Unknown |
 | [CitySDK](http://www.citysdk.eu/citysdk-toolkit/) | Open APIs for select European cities | No | Yes | Unknown |
 | [Daum Maps](http://apis.map.daum.net/) | Daum Maps provide multiple APIs for Korean maps | `apiKey` | No | Unknown |