Add source link to nuget packages.

[dam5s]

While source link is advertised for debugging purposes,
it would allow browsing the source code instead of having to manually search for the right documentation. Jetbrains Rider has source link integration built-in and will navigate to the source
when trying to navigate to the definition of a given type or function.

Here is the link on how to add it to a given fsproj file.
https://github.com/dotnet/SourceLink#using-sourcelink

I'm not super familiar with how project files are built but I'm guessing there might be a way to add this to all solution projects from just one place? If anyone can point me at the right location to put that config, I'm happy to do a pull request.

[dam5s]

#439