Added note to refer users to sgkit

[jeromekelleher]

As discussed offline with @alimanfoo, this is a note to point interested users towards sgkit. I don't think it's quite right at the moment, but here's a draft to get us started.

A couple of points:

• "Deprecation notice" seemed too strong, but I couldn't think of anything better
• Do we want to make concrete recommendations about who should use sgkit over sckit-allel?

cc @tomwhite @eric-czech @hammer

[jeromekelleher]

Thanks @alimanfoo, suggestions applied!

[alimanfoo]

Great, thanks @jeromekelleher.