porters handbook: De-empasize the use of shar

De-empasize the use of shar archives when submitting new ports.
When we were still using gnats and CVS, shar was the best way of submitting
new ports to the PR database, for inclusion in the ports tree. These days,
it is usually easier to use a patch from svn, or even git.

Move the shar example further to the bottom of the page in the section about
submitting ports to the FreeBSD project.

Dicussed with: many
Reviewed by: allanjude, bapt, bcr, emaste, mat
Differential Revision: https://reviews.freebsd.org/D25148