rake -T is useful

This adds the desc to each rake task so rake -T gives useful information.

Change-Id: I5af5d18361f481c36d853b9fb474c0dcaa942b1a

Rakefile

@@ -2,22 +2,27 @@ task default: ["test"]
 
 task :test => [:lint, :style, :unit]
 
+desc "Vendor the cookbooks in the Berksfile"
 task :berks_prep do
   sh %{chef exec berks vendor}
 end
 
+desc "Run FoodCritic (lint) tests"
 task :lint do
   sh %{chef exec foodcritic --epic-fail any --tags ~FC003 --tags ~FC023 .}
 end
 
+desc "Run RuboCop (style) tests"
 task :style do
   sh %{chef exec rubocop}
 end
 
+desc "Run RSpec (unit) tests"
 task :unit => :berks_prep do
   sh %{chef exec rspec --format documentation}
 end
 
+desc "Remove the berks-cookbooks directory and the Berksfile.lock"
 task :clean do
   rm_rf [
     'berks-cookbooks',