Add documentation for Avatar (#1469)

* add documentation for avatar

* add swiftui usage example

ios/docs/Controls/Avatar.md

@@ -0,0 +1,47 @@
+# Avatar
+
+## Overview
+`Avatar` is a graphical representation of a user, team, or entity. It can display an image, icon, or initials, and can be either circular or square.
+
+The different variations of `Avatar` are pictured below.
+
+#### Circle
+| Image | Initials | Icon | Accent Icon | Outline Icon | Outlined Primary Icon | Overflow |
+| - | - | - | - | - | - | - |
+| ![Avatar-Image.png](.attachments/Avatar-Image.png) | ![Avatar-Initials.png](.attachments/Avatar-Initials.png) | ![Avatar-Icon.png](.attachments/Avatar-Icon.png) | ![Avatar-AccentIcon.png](.attachments/Avatar-AccentIcon.png) | ![Avatar-OutlinedIcon.png](.attachments/Avatar-OutlinedIcon.png) | ![Avatar-OutlinedPrimaryIcon.png](.attachments/Avatar-OutlinedPrimaryIcon.png) | ![Avatar-Overflow.png](.attachments/Avatar-Overflow.png) |
+
+#### Square
+| Image | Initials |
+| - | - |
+| ![Avatar-ImageSquare.png](.attachments/Avatar-ImageSquare.png) | ![Avatar-InitialsSquare.png](.attachments/Avatar-InitialsSquare.png) |
+
+## Usage
+### UIKit
+```Swift
+let avatar = MSFAvatar(style: avatarStyle,
+                       size: avatarSize)
+let avatarState = avatar.state
+avatarState.primaryText = avatarPrimaryText
+avatarState.image = avatarImage
+```
+
+### SwiftUI
+```Swift
+Avatar(style: style,
+       size: size,
+       image: image,
+       primaryText: primaryText,
+       secondaryText: secondaryText)
+```
+
+## Implementation
+### Control Name
+`Avatar` in Swift, `MSFAvatar` in Objective-C
+
+### Source Code
+- [Avatar.swift](https://github.com/microsoft/fluentui-apple/blob/main/ios/FluentUI/Avatar/Avatar.swift)
+- [MSFAvatar.swift](https://github.com/microsoft/fluentui-apple/blob/main/ios/FluentUI/Avatar/MSFAvatar.swift)
+
+### Sample Code
+- [AvatarDemoController.swift](https://github.com/microsoft/fluentui-apple/blob/main/ios/FluentUI.Demo/FluentUI.Demo/Demos/AvatarDemoController.swift)
+- [AvatarDemoController_SwiftUI.swift](https://github.com/microsoft/fluentui-apple/blob/main/ios/FluentUI.Demo/FluentUI.Demo/Demos/AvatarDemoController_SwiftUI.swift)