update demo instructions

Author: Plamen5kov

demo/README.md

@@ -0,0 +1,7 @@
+This NativeScript project demonstrates how the plugin works.
+If you want to test it out on an emulator or a device you can follow the instructions below:
+
+* `git clone https://github.com/NativeScript/nativescript-angular.git`
+* `cd nativescript-angular/demo`
+* `npm run build.plugin && npm install`
+* `tns run android` or `tns run ios` depending on the platform you want to test