[Fixed] Download: Add a "Downloading jQuery using npm" section

[mgol]

jQuery now uses npm to internally manage its dependencies instead of Bower, jQuery plugins are now advised to get published to npm etc. and yet on the download page we still only advise Bower. We should add a section about using npm to get jQuery.

[dmethvin]

This was fixed by #123 right?

[mgol]

@dmethvin Yeah, this issue will auto-close once the jquery-3 branch is merged to master.

[mgol]

Look at the open PRs, some of them start with [Merged], same story.