README: Ensure uniqueness of temporary directory
README: Ensure uniqueness of temporary directory

When working with temporary files and especially in the `/tmp` directory, it’s better to ensure the file name uniqueness by using `mktemp` instead of simply creating files with predefined names.

See `mktemp(1)`: http://linux.die.net/man/1/mktemp

Ref. #3.

deanhume/grunt-zopfli