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/24350
Reviewed-by: Ian Lance Taylor <[email protected]>

CONTRIBUTORS

@@ -155,6 +155,7 @@ Benny Siegert <[email protected]>
 Benoit Sigoure <[email protected]>
 Berengar Lehr <[email protected]>
 Bill Neubauer <[email protected]> <[email protected]> <[email protected]>
+Bill O'Farrell <[email protected]>
 Bill Thiede <[email protected]>
 Billie Harold Cleek <[email protected]>
 Bjorn Tillenius <[email protected]>
@@ -548,6 +549,7 @@ Kai Backman <[email protected]>
 Kamal Aboul-Hosn <[email protected]>
 Kamil Kisiel <[email protected]> <[email protected]>
 Kang Hu <[email protected]>
+Karan Dhiman <[email protected]>
 Kato Kazuyoshi <[email protected]>
 Katrina Owen <[email protected]>
 Kay Zhu <[email protected]>
@@ -858,6 +860,7 @@ Ryan Seys <[email protected]>
 Ryan Slade <[email protected]>
 S.Çağlar Onur <[email protected]>
 Salmān Aljammāz <[email protected]>
+Sam Ding <[email protected]>
 Sam Hug <[email protected]>
 Sam Thorogood <[email protected]> <[email protected]>
 Sam Whited <[email protected]>
@@ -957,6 +960,7 @@ Totoro W <[email protected]>
 Travis Cline <[email protected]>
 Trevor Strohman <[email protected]>
 Trey Tacon <[email protected]>
+Tristan Amini <[email protected]>
 Tudor Golubenco <[email protected]>
 Tyler Bunnell <[email protected]>
 Tyler Treat <[email protected]>
@@ -999,6 +1003,8 @@ Yissakhar Z. Beck <[email protected]>
 Yo-An Lin <[email protected]>
 Yongjian Xu <[email protected]>
 Yoshiyuki Kanno <[email protected]> <[email protected]>
+Yu Heng Zhang <[email protected]>
+Yu Xuan Zhang <[email protected]>
 Yuki Yugui Sonoda <[email protected]>
 Yusuke Kagiwada <[email protected]>
 Yuusei Kuwana <[email protected]>