Add documentation for how packaging works

Now that we have settled out how packaging should work for both FOSS and PE, it would help to have some documentation on which files are used for which platform.

This could also involve some shuffling of files to improve organization, since .env, .service, .sysconfig, and .init files all live in a flat directory right now.

In Scope

• Adding ext/README.md that explains which platforms use each file
  • Optional: How a module like pe_razor could influence the variables used by the service.