docs: improve package maintainer instructions

[Eric Blake]

I noticed that the main link on the cygwin.com left navbar
(https://cygwin.com/setup.html#submitting) has outdated instructions;
rather than duplicate things, I'd rather have a link to the more
up-to-date page
(https://sourceware.org/cygwin-apps/package-upload.html).  Okay to push?

2014-08-02  Eric Blake  <eblake@redhat.com>

	* setup.html: Modernize, point to package-upload.html

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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