Adds documentation for rename option

[0x-r4bbit]

Fixes #118

[0x-r4bbit]

Okay guys, turned out I was wrong. Thanks to @ajoslin for pointing me to the core docs.

See here for reference: https://twitter.com/andytjoslin/status/407895289609981953

Feel free to close that PR. But maybe we can somehow make it a bit clearer that some options can be found in the core configuration guide.

[ajoslin]

+1 I think it would be good to point to the core documentation for files for 'more power' - as grunt-contrib-copy is really the only time I used the advanced building files object thingy (whatever you call it!)

[shama]

Thanks for the PR. I agree it would be good to add a note to all the contrib-tasks about the options available to all Grunt tasks. I've opened an issue for that on grunt-contrib-internal as that is the task that generates our readme files. See: gruntjs/grunt-contrib-internal#12