docs: Explain how to update with RAUC
Change-Id: Id009aed046ba00a448d7da7c0f2306e4b20dd45d
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It is *not* safe to perform incremental builds after changing an "important" setting.
Please check their manual for details.
Using `ccache` might help, but a significant time is wasted in `configure` steps which are not parallelized :( as of October 2017.
+
+## Installing updates to a device
+
+Apart from the traditional way of re-flashing the SD card or the eMMC from scratch, it's also possible to use RAUC to update.
+This method preserves the U-Boot version and the U-Boot's environment.
+Apart from that, everything starting with the kernel and the DTB file and including the root FS is updated.
+
+FIXME: the system uses separate config partitions (`/cfg`), so these persistent bits are *not* preserved yet (see these
+[user](https://tree.taiga.io/project/jktjkt-czechlight/us/124?no-milestone=1)
+[stories](https://tree.taiga.io/project/jktjkt-czechlight/us/127)).
+
+To install an update:
+
+```sh
+# build node
+make
+rsync -avP images/update.raucb somewhere.example.org:path/to/web/root
+
+# target, perhaps via an USB console
+wget http://somewhere.example.org/update.raucb -O /tmp/update.raucb
+rauc install /tmp/update.raucb
+reboot
+```