Include npmrc instructions with npm instructions if there's no npmrc

[sparr]

The help for ~/.npm/ says to add lines to your npmrc, but if you don't have one then you might naively created it at ~/.npmrc and then have to run another round of xdg-ninja to get the instructions for exporting NPM_CONFIG_USERCONFIG and moving it.

I propose that the .npmrc instructions should be included along with the .npm instructions if ~.npmrc and $NPM_CONFIG_USERCONFIG both don't exist.