Add "datestyle" info to installation guide

[DavidBlaa]

• postgresql.config: datestyle = 'iso, mdy'
• or better check datestyle during initial BEXIS installation and give a warning :-)

[DavidBlaa]

https://github.com/BEXIS2/Core/wiki/Primary-data-with-date-cannot-be-uploaded-even-though-validation-was-successful