Describe what Misen is and how to use it in README

README.md

@@ -1 +1,57 @@
 # Misen
+Misen is a script to support using Xcode Asset Catalog in Swift.  
+
+# Features
+Misen scans sub-directories in the specified Asset Catalog and creates a UIImage extension file which has the following features.
+- Application-specific enums which is constructed from Asset Catalog names.
+- UIImage non-failable initializer whose argument is an enum value above.
+
+#### e.g.
+Misen creates the UIImage extension file from the asset catalog with 3 image sets below.
+
+<img src="images/asset-catalog.png">
+
+```
+extension UIImage {
+
+    convenience init!(assetName: AssetName) {
+        self.init(named: assetName.rawValue)
+    }
+
+    enum AssetName: String {
+        case Camera = "Camera"
+        case Home = "Home"
+        case Mail = "Mail"
+    }
+}
+```
+
+- In your code, you can instantiate images in Asset Catalog as follows.
+
+```
+let camera = UIImage(assetName: .Camera)
+let home   = UIImage(assetName: .Home)
+let mail   = UIImage(assetName: .Mail)
+```
+
+
+# Usage
+- Change file permissions first.
+
+```
+chmod +x misen.swift
+```
+- Run the script.
+ - ```-path``` is a path of the asset catalog.
+ - ```-exportPath``` is an output UIImage extension file path.
+
+```
+./misen.swift -path PATH -exportPath OUTPUT_FILE_PATH
+```
+
+# Requirements
+- Xcode 6.3
+- Swift 1.2
+
+# License
+Misen is released under the MIT license. See LICENSE for details.