Document changes and release 5.12.2pureos4 of linux-librem5

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
2 jobs for release5122_4 in 63 minutes
Status Job ID Name Coverage
  Build
passed #353664
librem5
build:aarch64

00:22:30

passed #353665
librem5:arm64
build:deb

01:03:00