Added documentation on auto-update

auto-update.rst

+About auto update system
+************************
+
+Auto-update system is composed of two script:
+- slapudate.py in charge of updating/upgrading computer
+- upload-update.py in charge of uploading update information
+
+
+slapupdate
+----------
+slapudate is called by a cron job every day at midnight.
+
+It will connect to shacache to obtain last update information available.
+This file give authorized reporitories, date of needed reboot and upgrade
+and requested openSUSE version.
+
+If update date is newer than last requested update:
+1. We remove all repositories
+2. We add authorized repositories
+3. We check openSUSE version
+- If requested version is higher than current version we do a dist-upgrade
+  and reboot
+- Else we run upgrade
+
+If reboot date is newer than last requested one on we reboot.
+
+
+upload-update
+-------------
+
+This script will use a slapos-upgrade information file
+(in /etc/slapos-cache/slapos-upgrade by default),
+modify it according to option, ask confirmation and upload it to shacache.
+
+- Note: You will need your slapos.cfg for shacache upload ready in /etc/slapos-cache/
+
+options
++++++++
+  --upgrade-file=UPGRADE_FILE
+                        File use as reference to upgrade.
+  -u, --upgrade         If selected will update tomorrow.
+  -r, --reboot          If selected will reboot tomorrow.
+  -n, --dry-run         Simulate the execution steps
+  -h, --help            show this help message and exit
+
+slapos-upgrade information file
++++++++++++++++++++++++++++++++
+[repositories]
+suse = http://download.opensuse.org/distribution/12.1/repo/oss/
+slapos = http://download.opensuse.org/repositories/home:/VIFIBnexedi/openSUSE_12.1
+
+[system]
+reboot = 2012-09-05
+upgrade = 2012-09-05
+opensuse_version = 12.1
+
+Example
++++++++
+
+- We want computers to update (at midnight)
+python upload-update.py -u
+
+- We want computers to update and reboot (at midnght)
+python upload-update.py -u -r
+