Update appveyor.yml

Author: SimonCropp

src/appveyor.yml

@@ -5,6 +5,7 @@ environment:
   DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
 build_script:
 - pwsh: |
+    choco install pandoc
     Invoke-WebRequest "https://dot.net/v1/dotnet-install.ps1" -OutFile "./dotnet-install.ps1"
     ./dotnet-install.ps1 -JSonFile src/global.json -Architecture x64 -InstallDir 'C:\Program Files\dotnet'
 - dotnet build src --configuration Release