Document changes and release 5.13.19pureos4 of linux-librem5

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
2 jobs for release51319_4 in 80 minutes and 29 seconds (queued for 3 minutes and 5 seconds)
latest
Status Name Job ID Coverage
  Build
passed build:aarch64 #376027
librem5

00:16:26

passed build:deb #376028
librem5:arm64

01:12:05