Updating README to add a note to explain a usecase

I'm currently re-using bower config files for all my grunt "apps" and I had an issue where some of my apps don't use bower and don't have bower.json files. To get this task to continue in those cases without an error i just needed to set **copy** to false. 

I hope i explained the use case in the change, but I mainly wanted to start the ball rolling for this docs update. I am not usually the best at wording these things ;)