doc: add Support section in README

Add a Support section, borrowing heavily from wp-cli project.

Move stuff about contributing to Node.js to the bottom as vastly more
users are interested in using Node.js and getting help with Node.js than
contributing to Node.js. Information still belongs, just not at the top.
(Many people will know to look in CONTRIBUTING.md anyway.)

PR-URL: https://github.com/nodejs/node/pull/16533


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>