Add database design

 Changes to be committed:
	new file:   doc/database_design/incomeoutlay.PNG
	new file:   doc/database_design/job.PNG
	new file:   doc/database_design/priority.PNG
	new file:   doc/database_design/province.PNG
	new file:   doc/database_design/sex.PNG
	new file:   doc/database_design/table.PNG
	new file:   doc/database_design/type.PNG
	new file:   doc/database_design/user.PNG
	new file:   doc/database_design/user_have_type.PNG