Update the readme for pxe::installer usage

[zachfi]

Without this work, users are still led to believe the permute is the preferred way to create PXE installers with this module. The permute function was written in a time before iteration. As such, the new language constructs have replaced the need for any to ever use the permute function again.

Here we update our documentation to provide a modern example of how resource could be generated without the use of the permute function.

[zachfi]

I should also update the code references for the new style. Perhaps keep the permute for older Puppet versions, but at least add new ones, and make a note about the iteration method being preferred on Puppet 4.