Release notes are available here:
Docs are available at Unity - Manual: Unity User Manual (5.6)
Upgrade guide is available at https://docs.google.com/document/d/1KbRgmLuBhDsf7RnNrXz1ObOIq8r0-fIialmvqzCfYC4/edit?usp=sharing
(Note that online documentation is updated at a different rate from the release)
Please remember to back up your projects before opening them in a beta build.
Best Regards,
Carlos M Rincón