added contributing file

CONTRIBUTING.md

@@ -0,0 +1,7 @@
+### Contribution
+
+ABP is an open source platform.
+
+* Open a [new issue](https://github.com/volosoft/volo/issues/new) if you found a bug or if you have a new feature/enhancement idea.
+* Open a pull request if you want to make a development. Please create an issue before a development, so we can discuss it.
+* Contribute to the [documentation](docs/Index.md).