docs: update howto-contribute.txt

docs: update howto-contribute.txt

This patch does not change any wording or grammar. It
only shuffles the order of things and adds a table of
contents. For example: it moves coding related bullet
points into the Coding Style Chapter; it groups email
related Chapters together, and so fourth.

Signed-off-by: J William Piggott <elseifthen@gmx.com>