make the docs advise using yarn instead of npm?

[andrewljohnson]

I ran into an error trying to run the site locally using npm (on Mac Mojave).

error There was a problem loading the local develop command. Gatsby may not be installed in your site's "node_modules" directory. Perhaps you need to run "npm install"? You might need to delete your "package-lock.json" as well.

The easiest advice on the internet to avoid the issue is to use yarn. Is it desirable for me to submit a PR changing the readme to use yarn?

[lacker]

It would be okay to suggest both possibilities in the readme. I don't want to only suggest yarn, without more understanding of what exactly is going wrong here. There isn't too much weird going on here, it's a pretty normal Gatsby static site.