Docs: add badge for security vulnerabilities stat (#5)

Author: lirantal

README.md

@@ -2,7 +2,7 @@
 
 # Vest - Validation Testing
 
-[![npm version](https://badge.fury.io/js/vest.svg)](https://badge.fury.io/js/vest) [![Build Status](https://travis-ci.org/ealush/vest.svg?branch=master)](https://travis-ci.org/ealush/vest)
+[![npm version](https://badge.fury.io/js/vest.svg)](https://badge.fury.io/js/vest) [![Build Status](https://travis-ci.org/ealush/vest.svg?branch=master)](https://travis-ci.org/ealush/vest) [![Known Vulnerabilities](https://snyk.io/test/npm/vest/badge.svg)](https://snyk.io/test/npm/vest)
 
 - [Documentation homepage](https://ealush.github.io/vest)
 - **Try vest live**