Add "gh repo create" to SETUP.md

merges manubot/rootstock#419
closes manubot/rootstock#418

Co-authored-by: Daniel Himmelstein <daniel.himmelstein@gmail.com>
Co-authored-by: Anthony Gitter <agitter@users.noreply.github.com>

SETUP.md

@@ -89,7 +89,8 @@ git remote set-url origin https://github.com/$OWNER/$REPO.git
 git remote set-url origin git@github.com:$OWNER/$REPO.git
 ```
 
-Next, you must manually create an empty GitHub repository at <https://github.com/new>.
+Then create an empty repository on GitHub. 
+You can do this at <https://github.com/new> or via the [GitHub command line interface](https://github.com/cli/cli) (if installed) with `gh repo create`.
 Make sure to use the same "Owner" and "Repository name" specified above.
 Do not initialize the repository, other than optionally adding a Description.
 Next, push your cloned manuscript: