hgext: more patchbomb documentation + Add a description of how to enable this extension + Note which command it provides (it's not very easy to guess that "hgext.patchbomb" enables the "email" command, unless one can read Python sources) + Expand the descriptions of the -n and -m options of "hg email". + Mention that formail is (commonly) part of the procmail package.

#header#
# HG changeset patch
# User #author#
# Date #date|hgdate#
# Node ID #node#
#parent%changesetparent#
#desc#

#diff#