add AppVeyor badge

Author: Vladimir Kotal

README.md

@@ -7,6 +7,7 @@ Copyright (c) 2006, 2018 Oracle and/or its affiliates. All rights reserved.
 [![wercker status](https://app.wercker.com/status/e2a879f4677e679c0bbf8fa82bcc0dba/s/master "wercker status")](https://app.wercker.com/project/byKey/e2a879f4677e679c0bbf8fa82bcc0dba)
 [![Coverage status](https://coveralls.io/repos/oracle/opengrok/badge.svg?branch=master)](https://coveralls.io/r/oracle/opengrok?branch=master)
 [![SonarQube status](https://sonarcloud.io/api/project_badges/measure?project=org.opengrok%3Aopengrok-top&metric=alert_status)](https://sonarcloud.io/dashboard?id=org.opengrok%3Aopengrok-top)
+[![AppVeyor status](https://ci.appveyor.com/api/projects/status/8oqlk5yx2c5fnwqw/branch/master?svg=true)](https://ci.appveyor.com/project/vladak/opengrok-b5hnp/branch/master)
 
 1. [Introduction](#1-introduction)
 2. [OpenGrok setup](#2-opengrok-install-and-setup)