Wine Staging is an open source project licensed under the LGPL2 license (with some exceptions, see here). The source code is currently hosted on GitHub. Please note that this repository only contains patches. In order to build Wine Staging, you'll also need the source of the corresponding Wine development release. The repository also contains a couple of helper scripts to simplify the steps of applying all or a subset of the patches. Detailed instructions how to build Wine Staging are available in our packaging instructions.
Wine Staging mainly concentrates on experimental features and patches which are difficult to get into the development branch. If you have a very simple bug fix including tests, there is usually no need to send it to Wine Staging. You can directly contribute it to the development branch. However, if you already tried that without success, or are working on such a complex area that you do not really think its ready for inclusion, you might want to submit it to our Staging tree. Please open a patch submission request on bugs.wine-staging.com including the patch. More information is also available in our Wiki.