The Walking Skeleton

Restore section in user guide (#14)

* Add saving data, faq and command summary section in user guide

* Add 'table of contents' and 'quick start'

* Update some minor formatting in quick start section

* Change 'contact' to 'task'

* Add faq link in content list

* Removed clear command.
Chang all instances of addressbook/this app/the app to Agendum.