skiboot-5.6.0 was released on Wednesday 24th May 2017. It is the new stable release of skiboot, taking over from the 5.5 release, first released on April 7th 2017. It is the first release done in a regular six week release cycle, mirroring that of op-build.
For how the skiboot stable releases work, see Skiboot stable tree rules and releases for details.
This release is a good level set of POWER9 support for bringup activities. If you are doing bringup, it is strongly suggested you continue to follow skiboot master.