Hi Kamil,
Thank you for your interest in Gonum. The Gonum community is quite small now, with less than 10 core developers (off the top of my head).
If you'd care to submit PRs correcting typos in documentation or fixing links on the page, that would be great! Such contribution is always appreciated.
I think the best way to get started developing code is to find some topic which is particularly interesting in you but not supported fully in Gonum, and start hacking. I started contributing to Gonum when I knew very little about Go (I am still a Go newbie), but got much help along the way. Just don't get discouraged if the review process takes longer than you expected, and use it as an opportunity to learn more about Go and about the project :) If you haven't done it yet, I recommend going through the official Go tutorials first:
https://golang.org/doc/tutorial/ (you don't need to know the database and REST stuff to contribute to Gonum, though).
Hope that helps,
Roman