message: add a description, repository
diff: @@ -4,6 +4,8 @@ version = "0.1.0"
  authors = ["Niko Matsakis <niko@alum.mit.edu>"]
  edition = "2018"
  license = "Apache-2.0/MIT"
  +repository = "https://github.com/nikomatsakis/salsa"
  +description = "A generic framework for on-demand, incrementalized computation"
  [dependencies]
  derive-new = "0.5.5"
  