Add some update instructions to the README

Everything is complicated for somebody who looks at this
for the first time, so lets explain the basics.

gnomeos/README-install.md

@@ -46,3 +46,21 @@ Finally, you can start gdm:
 
 $ /usr/sbin/gdm
 
+Updating
+--------
+
+You need to pull the latest updates to your repos (assuming that
+you have your repos in /ostree)
+
+$ ostree-pull --repo=/ostree/repo origin gnomeos-3.4-i686-runtime
+$ ostree-pull --repo=/ostree/repo origin gnomeos-3.4-i686-devel
+
+And then you need to update your branches: to point to the new
+stuff. That is a little cumbersome, and the easiest is to use
+the gnomeos-update-branches script that comes with ostree. The
+script currently assumes that the repository is in the current
+directory.
+
+$ su -
+# cd /ostree
+# ~/src/ostree/gnomeos/yocto/gnomeos-update-branches.sh