For rootski.io, you are finished "onboarding" when you've picked a project with Eric and finished it!
There is a list of "Onboarding Tasks" further down that will help you get there.
Once you've done this, you are officially a "Core Contributor" to "The Rootski Project" and you can (and should) list it on your Linkedin experience.
Put the start date as when you started onboarding. If a lengthy period of time has gone by since you contributed to Rootski.io. Put the end date as the time you got your most recent code changes reviewed and merged.
You can list yourself as a "Core Contributor" and/or use a job title more related to the type of projects you've worked on: