Remove npm installation method in contributing guide

[cvermand]

Since the app uses a yarn.lock file to track every dependency's version, we can only guarantee a reliable dev and prod environment when using yarn.

To avoid creating unsafe/unreliable environments, every mention of npm install, npm run .. etc should be removed from both the readme and the contributing guide.

To make npm installation reliable, we would need to provide a package-lock.json file. Nonetheless, if we decide to do so it would mean that both need to be kept updated which would require some further maintenance.