Add AppVeyor build status badge to README.md

Author: daviwil

README.md

@@ -1,6 +1,6 @@
 # PowerShell Language Support for Visual Studio Code
 
-[![Join the chat at https://gitter.im/PowerShell/vscode-powershell](https://badges.gitter.im/PowerShell/vscode-powershell.svg)](https://gitter.im/PowerShell/vscode-powershell?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Build status](https://ci.appveyor.com/api/projects/status/x2g1u375ih4w1xcc/branch/develop?svg=true)](https://ci.appveyor.com/project/PowerShell/vscode-powershell/branch/develop) [![Join the chat at https://gitter.im/PowerShell/vscode-powershell](https://badges.gitter.im/PowerShell/vscode-powershell.svg)](https://gitter.im/PowerShell/vscode-powershell?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 This extension provides rich PowerShell language support for [Visual Studio Code](https://github.com/Microsoft/vscode).
 Now you can write and debug PowerShell scripts using the excellent IDE-like interface