Add build status to README (#132)

README.md

@@ -1,3 +1,8 @@
+<img alt="Android" src="build/img/android.png" width="20px" height="20px" hspace="2px"/>![Android Build Status](https://filament-build.storage.googleapis.com/badges/build_status_android.svg)
+<img alt="Linux" src="build/img/linux.png" width="20px" height="20px" hspace="2px"/>![Linux Build Status](https://filament-build.storage.googleapis.com/badges/build_status_linux.svg)
+<img alt="macOS" src="build/img/macos.png" width="20px" height="20px" hspace="2px"/>![MacOS Build Status](https://filament-build.storage.googleapis.com/badges/build_status_mac.svg)
+<img alt="Windows" src="build/img/windows.png" width="20px" height="20px" hspace="2px"/>![Windows Build Status](https://filament-build.storage.googleapis.com/badges/build_status_windows.svg)
+
 ![Filament logo](docs/images/filament_logo_small.png)
 
 # Filament