Add README entry for ISO 3166-1 alpha-2 validation

README.md

@@ -89,6 +89,7 @@ Validator                               | Description
 **isISSN(str [, options])**             | check if the string is an [ISSN](https://en.wikipedia.org/wiki/International_Standard_Serial_Number).<br/><br/>`options` is an object which defaults to `{ case_sensitive: false, require_hyphen: false }`. If `case_sensitive` is true, ISSNs with a lowercase `'x'` as the check digit are rejected.
 **isISIN(str)**                         | check if the string is an [ISIN][ISIN] (stock/security identifier).
 **isISO8601(str)**                      | check if the string is a valid [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) date.
+**isISO8601(str)**                      | check if the string is a valid [ISO 3166-1 alpha-2](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2) officially assigned country code.
 **isISRC(str)**                         | check if the string is a [ISRC](https://en.wikipedia.org/wiki/International_Standard_Recording_Code).
 **isIn(str, values)**                   | check if the string is in a array of allowed values.
 **isInt(str [, options])**              | check if the string is an integer.<br/><br/>`options` is an object which can contain the keys `min` and/or `max` to check the integer is within boundaries (e.g. `{ min: 10, max: 99 }`). `options` can also contain the key `allow_leading_zeroes`, which when set to false will disallow integer values with leading zeroes (e.g. `{ allow_leading_zeroes: false }`). Finally, `options` can contain the keys `gt` and/or `lt` which will enforce integers being greater than or less than, respectively, the value provided (e.g. `{gt: 1, lt: 4}` for a number between 1 and 4).