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 Name Job ID Coverage
  Build
passed build:aarch64 #342988
librem5

00:20:22

passed build:deb #342989
librem5:arm64

01:03:54