0. Make it fun
This is different than your "Be welcoming": people are probably joining the project to fix am issue that matters to them, good documentation will help them fix the issue, a welcoming attitude will encourage to send the first patch, good feedback will make sure the patch works well and maybe a second one will come and so on. But once the contributor's issues with the product are solved, why would he stay? Maybe because is fun to be part of your community?
If you make a list of every FOSS tool you can think of, indeed, there is LightZone as there is also the more powerful RawTherapee. But on a list of "best tools", darktable beats them all (actually darktable should have been a "core" app, even if it does not have a Windows version).