Added macOS to description

package.json

 {
   "name": "react-native-webview",
-  "description": "React Native WebView component for iOS, Android, and Windows 10 (coming soon)",
+  "description": "React Native WebView component for iOS, Android, macOS, and Windows 10 (coming soon)",
   "main": "index.js",
   "typings": "index.d.ts",
   "author": "Jamon Holmgren <jamon@infinite.red>",