[ADD] display a warning related to a wkfl process

ex: if the user passes a sale order from the manual to the automatic workflow
process, it can be useful to inform him that a cron could automatically confirm
the order in a close future.
it warns the user via an onchange pop-up box.
[UPD] updates help on warning field
Merge pull request #15 from rdeheele/7.0-add_warning_field

7.0 add warning field