Provide appropriate messages to invalid inputs

When a user inputs an empty description or index,
or an index out of range, the user must be alerted.

Let's,
* check if a description is not entirely whitespace
* check if all indices are in the appropriate range
* alert the user if the conditions are not satisfied

Add a brief user guide

A user guide is needed for users to understand how to use
the chatbot's commands.

Let's add the command details and examples, together with
the Ui.png image, to the user guide.