[Owncloud] Upgrade instructions

[Arthur Schiwon]

Hey,

we have a page about upgrading owncloud [1], which is not much more than a 
stub however.

I like to expand it and make it more detailed  to give real instructions and 
increase number of successful upgrades :) Please find my draft here [2].

I appreciate feedback, edits in the pad and of course check for correctness.

Cheers
Arthur

[1] http://owncloud.org/support/upgrade/
[2] http://titanpad.com/Uiv2vNvwtY