Good idea, I've added some example account links to the page.
The relationship is:
Pixelfed accounts can follow Pixelfed accounts and Mastodon accounts
Mastodon accounts can follow Pixelfed accounts and Mastodon accounts
Everything can follow everything else (although text-only posts aren't visible on Pixelfed's current interface).
To follow a Pixelfed account on Mastodon, you can click follow on its account through the web app (for example @earth ).