docs(README): Add note about file/content urls (#358)

Author: jcesarmobile

README.md

@@ -148,6 +148,8 @@ Whether to use a dark styled keyboard on iOS
 
     * The default origin for requests from the iOS WebView is `ionic://localhost`. If `Hostname` and `iosScheme` preferences are set, then origin will be `iosSchemeValue://HostnameValue`.
 
+1. The WebView is not able to display images, videos or other files from file or content protocols or if it doesn't have protocol at all. For those cases use `window.Ionic.WebView.convertFileSrc()` to get the proper url.
+
 1. Replace any usages of `window.Ionic.normalizeURL()` with `window.Ionic.WebView.convertFileSrc()`.
 
     * For Ionic Angular projects, there is an [Ionic Native wrapper](https://beta.ionicframework.com/docs/native/ionic-webview):