Update read.me to "Polyglot Notebooks"

[KathleenDollard]

The read.me uses ".NET Interactive" instead of "Polyglot Notebooks".

You might also include a link

There are also missing images. I tried to copy that here, and they worked, so they are probably only broken on NuGet

[jonsequitur]

The terms mean slightly different things but I get that it might be unclear. I'll give this a read through.

I'm not sure why the images don't work on nuget.org. Presumably they need to be Markdown image tags.