Update Unity package doc file

Author: antja0

Documentation~/com.3d-group.unity-simple-notifications.md

@@ -1,3 +1,89 @@
-# Index
+# Unity simple notifications
 
-- table of contents goes here
+Simple but powerful UI notifications package for Unity game engine.
+
+- [How to use](#how-to-use)
+- [Performance and thread safety](#performance-and-thread-safety)
+- [Configuration](#configuration)
+
+<!-- toc -->
+
+## How to use
+
+1. Right click scene hierarchy
+2. Click UI/Notifications
+
+![HowTo](images/HowTo.png)
+
+Now you can send Notifications from your script like this:
+```c#
+Notifications.Send("Hello world");
+```
+
+![Notification](images/SimpleShowCase.gif)
+
+Specify type (changes notification color) and click events with optional parameters:
+```c#
+private void Error()
+{
+   Notifications.Send("Spooky error!", NotificationType.Error, OnClick);
+}
+
+public void OnClick()
+{
+    // Do something.
+}
+```
+There is also async overload:
+```c#
+await Notifications.SendAsync("Warning!", NotificationType.Warning);
+```
+
+## Performance and thread safety
+
+- Notifications are rate limited based on duplicates sent recently and max notifications queue length 
+- Notifications can be send from another thread. Creating GameObjects still always happens on main thread
+
+Here is how it looks when billion notifications are sent simultaneously from another thread:
+
+![Notification](images/PerformanceShowCase.gif)
+
+Try it yourself! Code:
+```c#
+using UnityEngine;
+using System.Threading;
+using Group3d.Notifications;
+
+public class TEST : MonoBehaviour
+{
+    private void Start()
+    {
+        new Thread(() =>
+        {
+            Thread.CurrentThread.IsBackground = true;
+            var r = new System.Random();
+            int counter = 0;
+            while (counter < 1000000000)
+            {
+                Notifications.Send($"Test {r.Next(0, 10000)}");
+                counter++;
+            }
+        }).Start();
+    }
+}
+
+```
+
+## Configuration
+
+Confurable from the inspector:
+
+![Inspector](images/Inspector.png)
+
+Custom notification prefab can be created and assigned in the inspector. If doing so, consider these things:
+- Prefab must have NotificationUI component (included in the package)
+- Prefab must have RectTransform component with anchors set to top-stretch
+
+If prefab is null, notification will be created dynamically.
+
+Optional Font parameter is only used when notification is created dynamically - if that is the only thing you want to change.