There aren't specification for C# 6.0 at the same level there used to be for previous versions.
The closest thing is the design notes for Roslyn on CodePlex (and moving forward on GitHub).
Here's a list of the C# features:
https://roslyn.codeplex.com/wikipage?title=Language%20Feature%20Status&referringTitle=Documentation
And here are the notes for the null-propagating operator for example:
https://roslyn.codeplex.com/discussions/540883
Since the language design team moved to GitHub you can continue following the Design Notes filter on the Roslyn repository to see C# 7.0 coming into existence:
https://github.com/dotnet/roslyn/labels/Design%20Notes