design/package-layering: New file

I keep explaining the vision verbally, let's write it down.

design/package-layering.md

@@ -0,0 +1,27 @@
+rpm-ostree should support layering packages on top of a base tree.  Use cases:
+
+* As a system administrator, I need to use a
+  profiling/debugging/tracing tool like perf, strace, nmap
+
+* As a system administrator, I prefer Emacs over vi/nano
+
+The design here is this:
+
+$ rpm-ostree add emacs
+
+This will
+
+* Resolve dependencies between the requested emacs package and what we have installed
+* Download all packages
+* Check out via hardlinks a *new* copy of the base tree
+* Unpack and install all requested layered RPMs on top.  This requires
+  that the %post scripts are whitelisted
+  http://lists.rpm.org/pipermail/rpm-maint/2014-April/003682.html
+* Commit that tree as a local branch
+* Deploy that branch for the *next* boot
+
+In this model, you will then have to reboot to install Emacs.  A
+future step for rpm-ostree will be applying partial live updates:
+http://blog.verbum.org/2014/02/26/ostree-rigorous-and-reliable-deployment/
+
+