added shield

added placeholder shield for build whilst setting up some proper CI.

README.md

@@ -4,6 +4,7 @@
 [![API](https://img.shields.io/badge/API-10%2B-orange.svg?style=flat)](https://android-arsenal.com/api?level=10s)
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![NuGet](https://img.shields.io/nuget/v/xamarin.android.showcaseview.svg?label=NuGet)](https://www.nuget.org/packages/xamarin.android.showcaseview/)
+![Build: Passing](https://img.shields.io/badge/Build-Passing-green.svg)
 
 _Xamarin.Android_ Native showcase view. An easy-to-use customizable show case view with circular reveal animation, ported from [FancyShowCaseView](https://github.com/faruktoptas/FancyShowCaseView) by [Faruk Toptaş](https://github.com/faruktoptas)