Add xmldoc to nugets

All nugets now have an .XML file included, with all the xmldoc. What is the use
of all our lovely xmldoc if no one is actually able to read it? :-D

Author: angularsen

UnitsNet.Serialization.JsonNet/UnitsNet.Serialization.JsonNet.Common.props

@@ -9,6 +9,7 @@
     <Copyright>Copyright (c) 2015 Andreas Gullberg Larsen</Copyright>
     <IncludeSymbols>true</IncludeSymbols>
     <IncludeSource>true</IncludeSource>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <RepositoryUrl>https://github.com/angularsen/UnitsNet</RepositoryUrl>
     <PackageIconUrl>https://raw.githubusercontent.com/angularsen/UnitsNet/ce85185429be345d77eb2ce09c99d59cc9ab8aed/Docs/Images/logo-32.png</PackageIconUrl>
     <PackageProjectUrl>https://github.com/angularsen/UnitsNet</PackageProjectUrl>

UnitsNet.WindowsRuntimeComponent/UnitsNet.WindowsRuntimeComponent.csproj

@@ -37,6 +37,7 @@
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
     <GenerateLibraryLayout>true</GenerateLibraryLayout>
+    <DocumentationFile>..\Artifacts\UnitsNet.WindowsRuntimeComponent\UnitsNet.XML</DocumentationFile>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|x86'">
     <PlatformTarget>x86</PlatformTarget>

UnitsNet/UnitsNet.Common.props

@@ -9,6 +9,7 @@
     <Copyright>Copyright (c) 2013 Andreas Gullberg Larsen</Copyright>
     <IncludeSymbols>true</IncludeSymbols>
     <IncludeSource>true</IncludeSource>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <RepositoryUrl>https://github.com/angularsen/UnitsNet</RepositoryUrl>
     <PackageIconUrl>https://raw.githubusercontent.com/angularsen/UnitsNet/ce85185429be345d77eb2ce09c99d59cc9ab8aed/Docs/Images/logo-32.png</PackageIconUrl>
     <PackageProjectUrl>https://github.com/angularsen/UnitsNet</PackageProjectUrl>