Document changes and release 5.11.15pureos1 of linux-librem5

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
2 jobs for release51115 in 63 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #342988
librem5
build:aarch64

00:20:22

passed #342989
librem5:arm64
build:deb

01:03:54