[BENTO-111] Added a .gitattributes and a note for Windows hosts

A .gitattributes was added to the repository to ensure `ks.cfg`, `preseed.cfg` and `*.sh` files have correct `LF` line endings and `*.bat` files have correct `CRLF` line endings. Also added a note about this in the `README` so people packaging boxes from Windows Hosts can fix the problem if they encounter it.

See [BENTO-111](https://tickets.opscode.com/browse/BENTO-111) for the rationale behind this.

Regards,
Matt