Add architecture overview to readme

[vcavallo]

          > Reading this now, i think a major thing i would like to see as a reader is a brief overview of the architecture. As-is, we jump straight from "this is what this is on a high level" into "here's Sire".

"Okay, this is an event-sourced operating function, cool, i can kind of make sense of that if i think about it. I wonder what it consists of, does the design make sense?" "Here's how you write apps in it!"

We don't have to go deep into how we do IO, but just something like the tower diagram i showed at LC and maybe say something about

* Sire bootstrapping itself from PLAN

* Rex

* (that the runtime acts as a kernel outside the system ("exokernel") which interprets calls to hardware completely opaquely, thus allowing the formal system state to be decoupled from any particular implementation).

Should i create a new issue for this? I think this PR is fine to merge as-is since it's a clear improvement.

plopping this updated tower diagram here for use later:

Originally posted by @vcavallo in https://github.com/operating-function/pallas/issues/9#issuecomment-2269791864