Add a contributing file to J2V8

To describe the contribution guidelines, a CONTRIBUTING.md
file was added to J2V8

CONTRIBUTING.md

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+# How to contribute
+
+Patches and third-party contributions are essential for keeping J2V8 great and
+helping to evolve this library. There are a few guidelines that we need
+contributors to follow so we have a chance to keep on top of things.
+
+## Code contributions
+
+Code contributions are greatly appreciated. We currently target V8 3.26, so
+all contributions must link against that version of V8. Code contributions
+should be accompanied by a set of unit tests. All tests are written in Java
+and executed using JUnit. See `src/test/java` for examples.
+
+## JNI layer
+
+The C++ layer should be kept as thin as possible. This results in a more
+verbose JNI layer, but it also moves the more complicated logic to Java.
+
+## J2V8 core
+
+J2V8 core contains the basic V8 bindings, represented as a hierarchy of
+Java Classes. The core contains all the classes found in `com.eclipsesource.v8`.
+J2V8 core should have no external dependencies other than JRE 1.6 and the JNI
+layer.
+
+## J2V8 Utils
+
+J2V8 utils contains a higher level of abstraction and provides a number of
+utilities for using J2V8 in a Java System. The J2V8 utils contains all
+the classes found in `com.eclipsesource.v8.utils`. The J2V8 utils should
+have no external dependencies other than JRE 1.6, and J2V8 core.
+
+## Contributor License Agreement (CLA)
+
+J2V8 was originally developed by Innoopract Informationssysteme GMBH for
+the [Tabris.js](https://tabrisjs.com) project. However, we believe that
+J2V8 is more broadly useful, which is why we Open Sourced the project.
+To help protect all J2V8 users, and to protect all contributions, we are asking
+that all contributors sign the CLA. A copy of the CLA can be found [here](https://www.clahub.com/agreements/eclipsesource/J2V8).
+
+## Submitting changes
+
+ * Sign the [Contributor License Agreement](https://www.clahub.com/agreements/eclipsesource/J2V8).
+ * Create a GitHub issue describing to capture the issue.
+ * Push your changes to a topic branch in your fork of the repository.
+ * Submit a pull request to J2V8.