Added warn note to docs about MySQL issues with 0000-00-00 date strings

MySQL accepts 0000-00-00 as a valid date but MySQLdb converts those
values into None. So there will be problems for instance if trying to
transport the data using dumpdata/loaddata.

This patch refs #6642 that has been closed as wontfix since this is a
particular problem of MySQL.
Merge pull request #926 from catalanojuan/adds-dates-containing-zeros-note-to-docs

Added warn note to docs about MySQL issues with 0000-00-00 date strings