Add Appveyor CI badge to the README (#723)

README.md

 # Poetry: Dependency Management for Python
 
-![Poetry build status](https://travis-ci.org/sdispater/poetry.svg)
-
 Poetry helps you declare, manage and install dependencies of Python projects,
 ensuring you have the right stack everywhere.
 
@@ -9,6 +7,9 @@ ensuring you have the right stack everywhere.
 
 It supports Python 2.7 and 3.4+.
 
+[![Unix Build Status](https://img.shields.io/travis/sdispater/poetry.svg?label=Unix)](https://travis-ci.org/sdispater/poetry)
+[![Windows Build Status](https://img.shields.io/appveyor/ci/sdispater/poetry.svg?label=Windows)](https://ci.appveyor.com/project/sdispater/poetry)
+
 ## Installation
 
 Poetry provides a custom installer that will install `poetry` isolated