Output XML docs for Perspex.Markup.

src/Markup/Perspex.Markup/Perspex.Markup.csproj

@@ -24,6 +24,7 @@
     <DefineConstants>DEBUG;TRACE</DefineConstants>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
+    <DocumentationFile>bin\Debug\Perspex.Markup.XML</DocumentationFile>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DebugType>pdbonly</DebugType>