What is a stable version of DD4T 2.0 (.NET) - Git branch URL to download the source code for my local reference. On GitHub I can see only some modules branches and not the whole source code branches.
If you go to https://github.com/dd4t you will find all sources separated by package names, for .NET you have:
Add to that one of the DD4T.Providers for the version of SDL Tridion you are using (or SDL Web 8 of course ;o), and in case you want to use the The DD4T RestService, then add that one too (plus I think you need one of the Dependency Injection options).
More details on getting started can be found here https://github.com/dd4t/DD4T.Core/wiki/1.0-Getting-started-with-.NET