CONTRIBUTORS: add Raul Silvera (Google CLA)

Raul wrote the pprof code in CL 153750043.

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/146450043

CONTRIBUTORS

@@ -496,6 +496,7 @@ Preetam Jinka <pj@preet.am>
 Quan Yong Zhai <qyzhai@gmail.com>
 Raif S. Naffah <go@naffah-raif.name>
 Raph Levien <raph@google.com>
+Raul Silvera <rsilvera@google.com>
 Rémy Oudompheng <oudomphe@phare.normalesup.org> <remyoudompheng@gmail.com>
 Richard Crowley <r@rcrowley.org>
 Richard Eric Gavaletz <gavaletz@gmail.com>