Add initial roadmap.md, update contributing.md

[haadcode]

Looks good 👍 One thing that isn't clear to me from the roadmap is the respective priorities for each goal. Are they sorted by priority (top-to-bottom)? Or can we add some indicator as to "we'll do this first" and "we'll do this later"?

[latenssi]

Looks good 👍 One thing that isn't clear to me from the roadmap is the respective priorities for each goal. Are they sorted by priority (top-to-bottom)? Or can we add some indicator as to "we'll do this first" and "we'll do this later"?

Yea, the order was stated on line 17 :) so they are sorted by priority top-to-bottom.

[haadcode]

LGTM 👍 One more thing to add, I think, would be to add a link to the README so that visitors can find the roadmap. Doesn't need to be in this PR necessarily, but we should do that nevertheless.

Good to merge from my side 👍