Update user guide to include date of last visit

[rayray39]

closes #85

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/commands/AddCommand.java	100.00% <ø> (ø)	8.00 <0.00> (ø)