Add AppVeyor badge for Windows build

README.md

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 # subjack
 
-[![Build Status](https://api.travis-ci.org/haccer/subjack.svg?branch=master)](https://travis-ci.org/haccer/subjack) [![Go Report Card](https://goreportcard.com/badge/github.com/haccer/subjack)](https://goreportcard.com/report/github.com/haccer/subjack) [![GitHub license](https://img.shields.io/github/license/haccer/subjack.svg)](https://github.com/haccer/subjack/blob/master/LICENSE)
+[![Build Status](https://api.travis-ci.org/haccer/subjack.svg?branch=master)](https://travis-ci.org/haccer/subjack) [![Build status](https://ci.appveyor.com/api/projects/status/dm8f2yyjcbn3j1cm?svg=true&passingText=Windows%20-%20OK&failingText=Windows%20-%20failed&pendingText=Windows%20-%20pending)](https://ci.appveyor.com/project/haccer/subjack) [![Go Report Card](https://goreportcard.com/badge/github.com/haccer/subjack)](https://goreportcard.com/report/github.com/haccer/subjack) [![GitHub license](https://img.shields.io/github/license/haccer/subjack.svg)](https://github.com/haccer/subjack/blob/master/LICENSE)
 
 subjack is a Hostile Subdomain Takeover tool written in Go designed to scan a list of subdomains concurrently and identify ones that are able to be hijacked. With Go's speed and efficiency, this tool really stands out when it comes to mass-testing. Always double check the results manually to rule out false positives.