Update README.md

- Add system requirements
- Add an installation guide for Homebrew Cask

Author: magicien

README.md

@@ -5,8 +5,18 @@ macOS QuickLook plugin for glTF files. (.gltf/.glb)
 
 ![ScreenShot2](https://github.com/magicien/GLTFQuickLook/blob/master/screenshot2.gif)
 
+## System Requirements
+
+- macOS 10.13 (High Sierra) or later
+
 ## Install
 
+### Using [Homebrew Cask](https://github.com/phinze/homebrew-cask)
+
+- Run `brew cask install gltfquicklook`
+
+### Manually
+
 1. Download **GLTFQuickLook_vX.X.X.zip** from [Releases](https://github.com/magicien/GLTFQuickLook/releases/latest).
 2. Put **GLTFQuickLook.qlgenerator** (in the zip file) into `/Library/QuickLook` (for all users) or `~/Library/QuickLook` (only for the logged-in user).
 3. Run `qlmanage -r` command to reload QuickLook plugins.