new package for Siril 0.9.0

[Vincent Hourdin <debian-siril@free-astro.vinvin.tf>]

Dear debian-astro members,
we are about to release the first stable release of siril, 0.9.0. The
previous version, 0.9.0-rc1 was finally not a release candidate but more
a beta, this new version having a lot of new features in 10 months of
work. Starting with this stable release, we will follow a more normal
release cycle.

I updated debian's git to have a working package for this upcoming
version. Could somebody check that everything has been done the proper
way and that the package is ready for release? We could then confirm
that the upstream package is ready for release as well.

Thank you very much,
Vincent