docs(readme): add isMimeType description

Author: Nicolas Coutin

README.md

@@ -99,6 +99,7 @@ Validator                               | Description
 **isLowercase(str)**                    | check if the string is lowercase.
 **isMACAddress(str)**                   | check if the string is a MAC address.
 **isMD5(str)**                          | check if the string is a MD5 hash.
+**isMimeType(str)**                     | check if the string matches to a valid [MIME type](https://en.wikipedia.org/wiki/Media_type) format
 **isMobilePhone(str, locale)**          | check if the string is a mobile phone number,<br/><br/>(locale is one of `['ar-AE', 'ar-DZ','ar-EG', 'ar-JO', 'ar-SA', 'ar-SY', 'cs-CZ', 'de-DE', 'da-DK', 'el-GR', 'en-AU', 'en-CA', 'en-GB', 'en-HK', 'en-IN',  'en-KE', 'en-NG', 'en-NZ', 'en-RW', 'en-SG', 'en-UG', 'en-US', 'en-TZ', 'en-ZA', 'en-ZM', 'en-PK', 'es-ES', 'et-EE', 'fa-IR', 'fi-FI', 'fr-FR', 'he-IL', 'hu-HU', 'it-IT', 'ja-JP', 'ko-KR', 'lt-LT', 'ms-MY', 'nb-NO', 'nn-NO', 'pl-PL', 'pt-PT', 'ro-RO', 'ru-RU', 'sk-SK', 'sr-RS', 'tr-TR', 'uk-UA', 'vi-VN', 'zh-CN', 'zh-HK', 'zh-TW']` OR 'any'. If 'any' is used, function will check if any of the locales match).
 **isMongoId(str)**                      | check if the string is a valid hex-encoded representation of a [MongoDB ObjectId][mongoid].
 **isMultibyte(str)**                    | check if the string contains one or more multibyte chars.