Add PackageCloud.io release file

Add PackageCloud.io release file

Version 1.0.4-1 will be released for RIC Amber release.

Packages, which will be now released, were pushed to staging area
by this Jenkins job execution:

https://jenkins.o-ran-sc.org/job/ric-plt-sdl-docker-ci-pc-merge-master/5/

Package releasing instructions for reference:

https://docs.releng.linuxfoundation.org/projects/global-jjb/en/latest/jjb/lf-release-jobs.html#packagecloud-release-files

Additionally, improved README.md file instructions based on received
feedback.

Signed-off-by: Rolf Badorek <rolf.badorek@nokia.com>
Change-Id: Ibc3cda11d127e877b5a97c83c377139e91f2a013