Create an upgrade tutorial

(Don't know if this should apply to xwiki.org documentation just crossed my mind today)

My upgrade list of steps contains more than 50 steps.
Sooner or later this process should be automated, but it definitely should start with documenting all necessary steps for properly upgrading your XWiki instance (especially in cases where one might loose his old settings).

Attached my upgrade checklist in XWiki 2.1 format for you to have the idea.