Hey Adam 
I can help answer that question.
Our integrations library is something we manually curate internally. There’s no process for being listed in this library because it’s not a public directory.
We decide which integrations to list based on our own research about what the most beneficial tools are for Ghost users, as well as from direct customer feedback.
You’re more than welcome to suggest integrations on this forum, which is a great place to see if other people in the community would find it useful too 