Add documentation XML to NuGet package

The addin contains XML comments for aliases, but doesn't ship with the documentation XML file. Therefore comments can't be shown on Cake website (e.g. https://cakebuild.net/extensions/cake-assemblyinforeflector/).
To have aliases documented on the Cake website the NuGet package needs to ship with the documentation XML in the NuGet package.

Inspect the addin's packaging and build configuration to find where the NuGet package contents are defined, then check how the XML documentation file is generated. Done means the package includes that XML file and the aliases can be displayed on the Cake website.

Written by the indexing model from the issue text.