line_statistics is not an executable

Also add documentation on it

tools/README.md

@@ -4,5 +4,4 @@ This is a collection of utilities used for Rails internal development.
 They aren't used by Rails apps directly.
 
   * `console` drops you in irb and loads this local Rails repos
-  * `line_statistics` gives lines-of-code counts for our codebase
   * `profile` profiles `Kernel#require` to help reduce startup times

tools/line_statistics

@@ -1,4 +1,5 @@
-#!/usr/bin/env ruby
+# Class used to calculates LOC for a provided file list.
+#
 # Example:
 #   files = FileList["lib/active_record/**/*.rb"]
 #   CodeTools::LineStatistics.new(files).print_loc