Configuring tasks: Demonstrate use of grunt.file.read()

Would be very useful to add in the fact you can call in a file which appends.
I.E: grunt.file.read('./javascript/account/custom.js')

see the following stakeoverflow question: http://stackoverflow.com/questions/38854998/dynamic-mapping-and-concat-with-grunt-uglify/38858781#38858781

This is missing from the documentation at: http://gruntjs.com/configuring-tasks#importing-external-data
