Update User Guide

[crazoter]

Made significant updates to the user guide which can be found at https://github.com/AY1920S1-CS2103-F10-1/main/blob/master/docs/UserGuide.adoc on GitHub and https://ay1920s1-cs2103-f10-1.github.io/main/UserGuide.html online.

Take note of the following:

1. Sections of the UG marked as [INCOMPLETE UG] or [Parameters not updated] are incomplete and need to be finished by the developer.
2. The user guide is an AsciiDoc format. I recommend using Visual Studio Code and installing the asciidoctor visual studio code extension. (https://marketplace.visualstudio.com/items?itemName=joaompinto.asciidoctor-vscode) to edit this document.
3. Note that in the current implementation of AB3, the index of an object is variable; in other words if you use find its index will change. In our implementation (as defined in the user guide), this index is non-variant; in other words, an item will always have the same index regardless of whether it was displayed via patient list or patient find. This is to facilitate other more complicated commands such as patient add medcon which require the user to select both a patient and a medical condition.

[crazoter]

part of #49