docs: improve package maintainer instructions

[Yaakov Selkowitz]

On Sat, 2014-08-02 at 07:27 -0600, Eric Blake wrote:
> 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 (https://cygwin.com/setup.html#submitting) has outdated instructions;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?

A few minor nits:

> +  would be 4.5.13-1, etc). Some packages also use a YYMMDD format for
                                                       ^^^^^^
YYYYMMDD


> -boffo-1.0-1.tar.bz2  boffo-1.0-1-src.tar.bz2  setup.hint
> +boffo-1.0-1.tar.xz  boffo-1.0-1-src.tar.z  setup.hint
                                      ^^^^^^
.tar.xz

Corinna will have to give the final ack.


Yaakov