CONTRIBUTORS: add people who contributed to s390x port (IBM CLA)

Add Bill O'Farrell (corporate CLA for IBM)
Add Karan Dhiman (corporate CLA for IBM)
Add Sam Ding (corporate CLA for IBM)
Add Tristan Amini (corporate CLA for IBM)
Add Yu Heng Zhang (corporate CLA for IBM)
Add Yu Xuan Zhang (corporate CLA for IBM)

Change-Id: I9ab15e33954afc2c208fc2e420a72c5a4d865f9b
Reviewed-on: https://go-review.googlesource.com/24350Reviewed-by: Ian Lance Taylor <iant@golang.org>

CONTRIBUTORS

@@ -155,6 +155,7 @@ Benny Siegert <bsiegert@gmail.com>
 Benoit Sigoure <tsunanet@gmail.com>
 Berengar Lehr <Berengar.Lehr@gmx.de>
 Bill Neubauer <wcn@golang.org> <wcn@google.com> <bill.neubauer@gmail.com>
+Bill O'Farrell <billo@ca.ibm.com>
 Bill Thiede <couchmoney@gmail.com>
 Billie Harold Cleek <bhcleek@gmail.com>
 Bjorn Tillenius <bjorn@tillenius.me>
@@ -548,6 +549,7 @@ Kai Backman <kaib@golang.org>
 Kamal Aboul-Hosn <aboulhosn@google.com>
 Kamil Kisiel <kamil@kamilkisiel.net> <kamil.kisiel@gmail.com>
 Kang Hu <hukangustc@gmail.com>
+Karan Dhiman <karandhi@ca.ibm.com>
 Kato Kazuyoshi <kato.kazuyoshi@gmail.com>
 Katrina Owen <katrina.owen@gmail.com>
 Kay Zhu <kayzhu@google.com>
@@ -858,6 +860,7 @@ Ryan Seys <ryan@ryanseys.com>
 Ryan Slade <ryanslade@gmail.com>
 S.Çağlar Onur <caglar@10ur.org>
 Salmān Aljammāz <s@0x65.net>
+Sam Ding <samding@ca.ibm.com>
 Sam Hug <samuel.b.hug@gmail.com>
 Sam Thorogood <thorogood@google.com> <sam.thorogood@gmail.com>
 Sam Whited <sam@samwhited.com>
@@ -957,6 +960,7 @@ Totoro W <tw19881113@gmail.com>
 Travis Cline <travis.cline@gmail.com>
 Trevor Strohman <trevor.strohman@gmail.com>
 Trey Tacon <ttacon@gmail.com>
+Tristan Amini <tamini01@ca.ibm.com>
 Tudor Golubenco <tudor.g@gmail.com>
 Tyler Bunnell <tylerbunnell@gmail.com>
 Tyler Treat <ttreat31@gmail.com>
@@ -999,6 +1003,8 @@ Yissakhar Z. Beck <yissakhar.beck@gmail.com>
 Yo-An Lin <yoanlin93@gmail.com>
 Yongjian Xu <i3dmaster@gmail.com>
 Yoshiyuki Kanno <nekotaroh@gmail.com> <yoshiyuki.kanno@stoic.co.jp>
+Yu Heng Zhang <annita.zhang@cn.ibm.com>
+Yu Xuan Zhang <zyxsh@cn.ibm.com>
 Yuki Yugui Sonoda <yugui@google.com>
 Yusuke Kagiwada <block.rxckin.beats@gmail.com>
 Yuusei Kuwana <kuwana@kumama.org>