docs: Add CONTRIBUTING note about conflicting/overlapping work

Adds some text to the contributing guide asking contributors to check on the ticket or IRC before starting on big contributions. In the past we've needed to close MRs where they conflict with active work by someone on the Arti team, but this can be discouraging to contributors. I think it's worth stating this explicitly in the guide so that we can be more upfront about expectations and hopefully avoid some of these situations in the future.

/cc @gabi-250 (I think I mentioned some time in the past that I wanted to document this somewhere.)