Remove npm references from Contributing

[afontcu]

Closes #1235 (sorta...)

Right now, npm is not a valid option as the project won't run properly. Only yarn is. Thus, I updated the docs accordingly.

In an ideal world, running VTU scripts with npm would "just work", but I'm not experienced enough with Lerna to identify what's missing. That's why I thought that, at least, we should make the docs reflect the current state of the art.

[lmiller1990]

Sounds good.

[lmiller1990]

Seems will still reference npm in various places, including release scripts and package.json. We might want to move all those to yarn as well. What do you think?

[afontcu]

Seems will still reference npm in various places, including release scripts and package.json. We might want to move all those to yarn as well. What do you think?

Hm, I didn't want to change the release scripts as they are working fine (AFAIK). Same thing with package.json scripts as long as they are run using yarn. Should we update them anyway?

[lmiller1990]

Your call - I think it makes sense to pick and stick w/ a single package manager. It's basically as easy as s/npm run/yarn/g, but we can do it in a separate PR if you are more comfortable with that too.

[lmiller1990]

Since moving the scripts from npm to yarn is (maybe) a more significant change, let's chat about it and do that in another PR. This is fine for now, nice job.