Add docs about `User not found`

[marwahaha]

(here for the socks!)

This reduces confusion seen in a few recent issues. https://github.com/npm/npm/issues/19432 https://github.com/npm/npm/issues/19404 https://github.com/npm/npm/issues/19387

It used to look like what's in the issues.

Now it would look like this (just changed the line under E404):

kmarwaha73-mac:yo-yo-yo kmarwaha$ npm publish .
npm ERR! publish Failed PUT 404
npm ERR! code E404
npm ERR! 404 User not found. Check `npm whoami` and make sure you have a NPM account.
npm ERR! 404 
npm ERR! 404  'yo-yo-y4' is not in the npm registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
npm ERR! 404 
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/kmarwaha/.npm/_logs/2017-12-23T07_07_28_739Z-debug.log
kmarwaha73-mac:yo-yo-yo kmarwaha$ 

This issue, I think, stems from new users "auth'd" on npm, but without an account made on the website.

Let me know how I can be useful here!