My Nexus 7 2013, running Android 6.0 MRA58K presently, the first public Marshmallow build is proposing to install an OTA.
As usual and unfortunately, there's no details or Changelog.
Shall I try to capture it?
Edit: Well that's actually very easy to do:
Here you go! This is MRA58U
Changes concern mostly the media (stagefright), camera and DRM frameworks.