Add LICENSE.txt to packable project items (#667)

Author: Shane32

Directory.Build.props

@@ -46,6 +46,7 @@
   <!-- Include README for packable projects -->
   <ItemGroup Condition="'$(IsPackable)' == 'true'">
     <None Include="$(MSBuildThisFileDirectory)README.md" Pack="true" PackagePath="\" />
+    <None Include="$(MSBuildThisFileDirectory)LICENSE.txt" Pack="true" PackagePath="\" />
     <None Include="$(MSBuildThisFileDirectory)nuget-icon.png" Pack="true" PackagePath="\" />
   </ItemGroup>