README: add appveyor build badge [ci skip]

Closes #2913

README.md

@@ -2,7 +2,8 @@
 
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/63/badge)](https://bestpractices.coreinfrastructure.org/projects/63)
 [![Coverity passed](https://scan.coverity.com/projects/curl/badge.svg)](https://scan.coverity.com/projects/curl)
-[![Build Status](https://travis-ci.org/curl/curl.svg?branch=master)](https://travis-ci.org/curl/curl)
+[![Travis-CI Build Status](https://travis-ci.org/curl/curl.svg?branch=master)](https://travis-ci.org/curl/curl)
+[![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/l1vv31029huhf4g4?svg=true)](https://ci.appveyor.com/project/curlorg/curl)
 [![Coverage Status](https://coveralls.io/repos/github/curl/curl/badge.svg)](https://coveralls.io/github/curl/curl)
 [![Backers on Open Collective](https://opencollective.com/curl/backers/badge.svg)](#backers)
 [![Sponsors on Open Collective](https://opencollective.com/curl/sponsors/badge.svg)](#sponsors)