README: Attempt to flesh out more, start moving docs from wiki

I'd like to migrate content from the GNOME wiki, as frankly the wiki
is crap.  Markdown in git is better in every way.

Start by fleshing out the README.md to be more useful.

docs-md/atomic-upgrades.md

@@ -0,0 +1,12 @@
+Atomic upgrades and rollback
+----------------------------
+
+Traditional package managers operate "live" on the running system.
+This means it's possible for interrupted updates to result in a
+half-updated system.  This model also makes it significantly harder to
+support rollbacks when updates fail.
+
+In contrast, OSTree always creates a *new* root whenever it's
+performing an update.  This new root shares storage via hardlinks with
+the current system.  Upon success, the bootloader configuration will
+be updated.