Add appveyor badge

Use master branch

README.Rmd

@@ -15,7 +15,7 @@ options(width = 120)
 ```
 
 [![Build Status](https://travis-ci.org/ropensci/skimr.svg?branch=master)](https://travis-ci.org/ropensci/skimr)
-[![Build status](https://ci.appveyor.com/api/projects/status/8p93v3t949ubyl06?svg=true)](https://ci.appveyor.com/project/michaelquinn32/skimr)
+[![Build status](https://ci.appveyor.com/api/projects/status/8p93v3t949ubyl06/branch/master?svg=true)](https://ci.appveyor.com/project/michaelquinn32/skimr/branch/master)
 [![codecov](https://codecov.io/gh/ropensci/skimr/branch/master/graph/badge.svg)](https://codecov.io/gh/ropensci/skimr)
 
 `skimr` provides a frictionless approach to summary statistics which conforms