The 1.12 release is tomorrow and the binaries are available now in the archives.
If you like, test rustup with
rustup update stable-2016-09-27
or rustup.sh with
curl -sSf https://static.rust-lang.org/rustup.sh | sh -s -- --spec=stable-2016-09-27
Changelog is here: https://github.com/rust-lang/rust/blob/master/RELEASES.md
I always found it surprising that the master branch does not have the release notes for the beta release. We already know what is going to be in the 1.13 release notes. Of course, we get excellent and clear release notes. If the persons volunteering to do them want to do it just before release... They are the ones volunteering, so it is up to them.