Fixed missing SK locale for isPostalCode

In source code you already support SK locale for few years, but it was missing in documentation. 
validator.js/src/lib/isPostalCode.js

Author: tfilo

README.md

@@ -154,7 +154,7 @@ Validator                               | Description
 **isOctal(str)**                        | check if the string is a valid octal number.
 **isPassportNumber(str, countryCode)**    | check if the string is a valid passport number.<br/><br/>(countryCode is one of `[ 'AM', 'AR', 'AT', 'AU', 'BE', 'BG', 'BY', 'BR', 'CA', 'CH', 'CN', 'CY', 'CZ', 'DE', 'DK', 'DZ', 'EE', 'ES', 'FI', 'FR', 'GB', 'GR', 'HR', 'HU', 'IE' 'IN', 'IR', 'ID', 'IS', 'IT', 'JP', 'KR', 'LT', 'LU', 'LV', 'LY', 'MT', 'MY', 'MZ', 'NL', 'PL', 'PT', 'RO', 'RU', 'SE', 'SL', 'SK', 'TR', 'UA', 'US' ]`.
 **isPort(str)**                         | check if the string is a valid port number.
-**isPostalCode(str, locale)**           | check if the string is a postal code,<br/><br/>(locale is one of `[ 'AD', 'AT', 'AU', 'AZ', 'BE', 'BG', 'BR', 'BY', 'CA', 'CH', 'CN', 'CZ', 'DE', 'DK', 'DO', 'DZ', 'EE', 'ES', 'FI', 'FR', 'GB', 'GR', 'HR', 'HT', 'HU', 'ID', 'IE' 'IL', 'IN', 'IR', 'IS', 'IT', 'JP', 'KE', 'KR', 'LI', 'LK', 'LT', 'LU', 'LV', 'MT', 'MX', 'MY', 'NL', 'NO', 'NP', 'NZ', 'PL', 'PR', 'PT', 'RO', 'RU', 'SA', 'SE', 'SG', 'SI', 'TH', 'TN', 'TW', 'UA', 'US', 'ZA', 'ZM' ]` OR 'any'. If 'any' is used, function will check if any of the locals match. Locale list is `validator.isPostalCodeLocales`.).
+**isPostalCode(str, locale)**           | check if the string is a postal code,<br/><br/>(locale is one of `[ 'AD', 'AT', 'AU', 'AZ', 'BE', 'BG', 'BR', 'BY', 'CA', 'CH', 'CN', 'CZ', 'DE', 'DK', 'DO', 'DZ', 'EE', 'ES', 'FI', 'FR', 'GB', 'GR', 'HR', 'HT', 'HU', 'ID', 'IE' 'IL', 'IN', 'IR', 'IS', 'IT', 'JP', 'KE', 'KR', 'LI', 'LK', 'LT', 'LU', 'LV', 'MT', 'MX', 'MY', 'NL', 'NO', 'NP', 'NZ', 'PL', 'PR', 'PT', 'RO', 'RU', 'SA', 'SE', 'SG', 'SI', 'SK', 'TH', 'TN', 'TW', 'UA', 'US', 'ZA', 'ZM' ]` OR 'any'. If 'any' is used, function will check if any of the locals match. Locale list is `validator.isPostalCodeLocales`.).
 **isRFC3339(str)**                      | check if the string is a valid [RFC 3339](https://tools.ietf.org/html/rfc3339) date.
 **isRgbColor(str [, includePercentValues])**                     | check if the string is a rgb or rgba color.<br/><br/>`includePercentValues` defaults to `true`. If you don't want to allow to set `rgb` or `rgba` values with percents, like `rgb(5%,5%,5%)`, or `rgba(90%,90%,90%,.3)`, then set it to false.
 **isSemVer(str)**                       | check if the string is a Semantic Versioning Specification (SemVer).