At time of writing (May 2016) Microsoft hasn’t yet finished updating the spec for C#6. In the meantime, I put up Microsoft’s latest current draft of the C#6 spec here: https://github.com/ljw1004/csharpspec/blob/gh-pages/README.md
This current draft is almost finished, save for a few remaining TODO comments and checking. (This version has been converted into github+markdown, but it also has links to download DOCX and PDF versions of the spec, and a link to download the grammar in ANTLR format).
For a complete list of all spec changes from C#5 to C#6, see the pull request: https://github.com/ljw1004/csharpspec/pull/1/files
(Why hasn’t Microsoft yet got around to officially releasing the C#6 spec? because of work in progress to align with ECMA, which of course takes time; it seemed a shame to expend effort on a release of the C#6 spec now only to have a subsequent version of the C#6 spec come out later after alignment with ECMA).