Update contributing.md which is a copy of how-to-contribute.md in https://github.com/apache/incubator-weex-site/pull/318
Update contributing.md which is a copy of how-to-contribute.md in https://github.com/apache/incubator-weex-site/pull/318

alibaba/weex,alibaba/weex,KalicyZhou/incubator-weex,alibaba/weex,alibaba/weex,KalicyZhou/incubator-weex,alibaba/weex,KalicyZhou/incubator-weex,KalicyZhou/incubator-weex,alibaba/weex,KalicyZhou/incubator-weex,KalicyZhou/incubator-weex,alibaba/weex