lib: document ProgressReport usage

lib/tasks/task_helper.rb

@@ -15,6 +15,17 @@ def rake_print(*args)
   end
 end
 
+# Display progress of a long-running Rake task.
+#
+# Usage:
+#
+# ```
+# progress = ProgressReport.new(100)
+# (0..100).times do
+#   progress.inc
+# end
+# progress.finish
+# ````
 class ProgressReport
   def initialize(total)
     @start = Time.zone.now