Linux 5.0 release brought us Adiantum file system encryption for fscrypt, support for AMDGPU FreeSync, Btrfs swap file support, Energy-aware Scheduling for Arm big.LITTLE, and many other changes. It was also the first release where I started to cover RISC-V changelog.
You can read a complete Linux 5.1 changelog with comments only here. It has been generated with git log v5.0..v5.1-rc7 --stat. For an easier to read more broad changelog, head over to KernelNewbies.