Update build and tool instructions regarding ILMerge

[Sebastian-Roth]

As it seems like ILMerge is not being provided by Microsoft as MSI installer anymore we need to work out a different way. It's available as project on github and we need to figure out how to use that within our build chain: https://github.com/dotnet/ILMerge

Or possibly use the nuget package: https://www.nuget.org/packages/ilmerge (it should include the binary as mentioned here)

For now people can use the old MSI provided here: https://github.com/mono/nuget/blob/master/Tools/ILMerge/ILMerge.msi