Additional troubleshooting section in CONTRIBUTING.md

@staltz, the developer can avoid this particular error by installing the node-sodium additional dependencies, so I think it would be a better idea to modify that part of the guide to make it mandatory on Linux too.

I got the error because I'm setting up the development environment on a new Ubuntu installation, so I imagine other people will have the same issue with other distributions if they are also new.

Should I modify the MR to change that dependencies part instead?