Updated the "Contributing" link on the cygwin site

[Christopher Faylor]

I have just updated the "Contributing" or "How do I help out?" link at
http://cygwin.com/contrib.html .

I've added more words about what is expected for ChangeLog entries and
diffs, removed some somewhat obsolete info, and most importantly, changed
all of the "Cygnus" entries to "Red Hat".

Please give this page a peek if you are planning on contributing code to
the Cygwin DLL or its helper utilities.

Thanks,
cgf