Add screenshot and brief User Guide

Duke has no representative screenshot and User Guide for the user to
view.

Let's add these so that the user can go through them and gain a better
understanding of how to use Duke.

docs/README.md

@@ -2,28 +2,35 @@
 
 ## Features 
 
-### Feature-ABC
+### Task Management
 
-Description of the feature.
+Let Duke manage your tasks for you! Simply add a task to your list and update it when completed.
 
-### Feature-XYZ
+### Multiple Types of Tasks
 
-Description of the feature.
+Want to add details to certain kinds of tasks by adding dates and events? We've 3 different kinds of tasks for you!
+
+### Simple Commands
+Duke functions on simple, easy-to-remember commands. Need help? Don't worry! We've a command for that too :grin:
 
 ## Usage
 
-### `Keyword` - Describe action
+### `list` - Lists all tasks
 
-Describe the action and its outcome.
+Lists all tasks and their current status
 
 Example of usage: 
 
-`keyword (optional arguments)`
+`list`
 
 Expected outcome:
 
-Description of the outcome.
+A numbered list of all tasks, e.g.
 
 ```
-expected output
+--------------------------------------------
+    Here are the tasks in your list:
+    1. [T][] task_desc
+    2. [D][X] task_desc (by: 2-5-2022)
+--------------------------------------------
 ```