Document @IsIso6391() in README.md

Author: RobinvanderVliet

README.md

@@ -874,6 +874,7 @@ isBoolean(value);
 | `@IsLatitude()`                                        | Checks if the string or number is a valid latitude coordinate.                                                                                                                                        |
 | `@IsLongitude()`                                       | Checks if the string or number is a valid longitude coordinate.                                                                                                                                       |
 | `@IsMobilePhone(locale: string)`                       | Checks if the string is a mobile phone number.                                                                                                                                                        |
+| `@IsISO6391()`                                         | Checks if the string is a valid ISO 639-1 officially assigned language code.                                                                                                                          |
 | `@IsISO31661Alpha2()`                                  | Checks if the string is a valid ISO 3166-1 alpha-2 officially assigned country code.                                                                                                                  |
 | `@IsISO31661Alpha3()`                                  | Checks if the string is a valid ISO 3166-1 alpha-3 officially assigned country code.                                                                                                                  |
 | `@IsLocale()`                                          | Checks if the string is a locale.                                                                                                                                                                     |