Added more documentation

Added documentation to the GUI
Merge branch 'sprint-3' of https://github.com/CIT275DevGroup/openTill into sprint-3

Conflicts:
	openTill/openTill.GUI/ManageInventory.xaml
	openTill/openTill.GUI/Views/InventoryLite.xaml
	openTill/openTill.GUI/Views/TransactionLite.xaml