macOS Sierra ALL features page
I wasn't able to find full, straight-from-the-Apple's-mouth release notes for Sierra, but you can find developer-level release notes for all versions of macOS since OS X 10.6 on this page on the Apple developer website. The notes for macOS Sierra 10.12 can be found here and those for 10.12.1 can be found here. However, note the caveat:

This document describes key developer-level features that were introduced in different versions of macOS.
This document is not intended as a complete list of features or changes for each new version of macOS. Instead, it focuses on those features or changes that affect the development of third-party software, providing overviews of each along with insight into how and when you might use them to create your own software.