doc: move the readme newcomers section

Move the newcomers section to be more noticeable to those who it will
be needed by.
Also removed some entries that are now effectively duplicates.
They are all in the above section.

PR-URL: https://github.com/nodejs/node/pull/6681


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>