add gradle information

Author: luigidr

README.md

@@ -7,3 +7,6 @@ Two simple projects, made in Eclipse (Neon), for getting started with the [OpenC
 Please, note that both projects are Eclipse projects, realized for teaching purposes. Before using any of them, you need to install the OpenCV library and create a `User Library` named `opencv` that links to the OpenCV jar and native libraries.
 
 A guide for getting started with OpenCV and Java is available at [http://opencv-java-tutorials.readthedocs.org/en/latest/index.html](http://opencv-java-tutorials.readthedocs.org/en/latest/index.html).
+
+## Gradle
+A version of this project that uses Gradle is available in the [gradle](https://github.com/opencv-java/getting-started/tree/gradle) branch.