#1 Add comments to Gruntfile.js and README.md

Originally at 2015-01-05T11:29:13Z by Hannes Hirzel closed at 2015-01-12T16:42:58Z For version 0.13 the ``Gruntfile.js`` had to be customized manually. Add some comments in the README.md file and in the ``Gruntfile.js`` how this had been done. See https://github.com/amber-smalltalk/amber/issues/1012#issuecomment-68696521