doc: add robertkowalski as collaborator

PR-URL: #977
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Christian Tellnes <christian@tellnes.com>
Reviewed-By: Rod Vagg <r@va.gg>

Author: robertkowalski

README.md

@@ -281,6 +281,7 @@ information about the governance of the io.js project, see
 * **Wyatt Preul** ([@geek](https://github.com/geek)) <wpreul@gmail.com>
 * **Brian White** ([@mscdex](https://github.com/mscdex)) <mscdex@mscdex.net>
 * **Christian Vaagland Tellnes** ([@tellnes](https://github.com/tellnes)) <christian@tellnes.com>
+* **Robert Kowalski** ([@robertkowalski](https://github.com/robertkowalski)) <rok@kowalski.gd>
 
 Collaborators follow the [COLLABORATOR_GUIDE.md](./COLLABORATOR_GUIDE.md) in
 maintaining the io.js project.