doc: add mcollina to collaborators

Hello everyone!

PR-URL: #4220
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Calvin Metcalf <calvin.metcalf@gmail.com>

Author: mcollina

README.md

@@ -402,6 +402,7 @@ information about the governance of the Node.js project, see
 * [julianduque](https://github.com/julianduque) - **Julian Duque** <julianduquej@gmail.com>
 * [JungMinu](https://github.com/JungMinu) - **Minwoo Jung** <jmwsoft@gmail.com>
 * [lxe](https://github.com/lxe) - **Aleksey Smolenchuk** <lxe@lxe.co>
+* [mcollina](https://github.com/mcollina) - **Matteo Collina** <matteo.collina@gmail.com>
 * [mhdawson](https://github.com/mhdawson) - **Michael Dawson** <michael_dawson@ca.ibm.com>
 * [micnic](https://github.com/micnic) - **Nicu Micleușanu** <micnic90@gmail.com>
 * [mikeal](https://github.com/mikeal) - **Mikeal Rogers** <mikeal.rogers@gmail.com>