Where is the best place to learn how to write shaders for Unity 5?

I have spent a good while looking through the documentation and at various shader files people have written, but I seem to keep coming across new and undocumented keywords/functions that people have used, seemingly pulling them out of thin air (such as 'UNITY_BRDF_GI').

Where are people learning the new shader system, that they know about the existence of these keywords and the like?

 

You can download the built in shaders for your version/editor platform, like the ones for windows. Then, you could try making a variant of the standard shader, if you want to get familiar with the lighting macros and keywords.

There's also: http://docs.unity3d.com/Manual/SL-BuiltinIncludes.html