Adds MSVC 2019 to appveyor build status description

Author: ryanhaining

README.md

@@ -14,7 +14,7 @@ Follow [@cppitertools](https://twitter.com/cppitertools) for updates.
 Status | Compilers
 ---- | ---- 
 [![Travis Build Status](https://travis-ci.com/ryanhaining/cppitertools.svg?branch=master)](https://travis-ci.com/ryanhaining/cppitertools) | gcc-7  gcc-8 clang-5.0 clang-6.0
-[![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/github/ryanhaining/cppitertools?svg=true)](https://ci.appveyor.com/project/ryanhaining/cppitertools) | MSVC 2017
+[![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/github/ryanhaining/cppitertools?svg=true)](https://ci.appveyor.com/project/ryanhaining/cppitertools) | MSVC 2017 MSVC 2019
 
 #### Table of Contents
 [range](#range)<br />