Document how to add migration in the core

[glpi-project]

Document how to add migration in the core

[edgardmessias]

Idea:

• Rename all files to update_to_XYZ (Ex. update_to_920, update_to_922, update_to_085)
• Rename all functions to updatetoXYZ
• Change the update.class.php to list all files ordered with update_to_*
• Each file, get the version in the name and compare it with the current version.