Add Peter Kasting to CONTRIBUTORS

"Google Inc." is already in AUTHORS.
diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index f0db3e4..db5812b 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -39,3 +39,4 @@
 Mike Kaufman <mkaufman@microsoft.com>
 Muhammad Aldo Firmansyah <TheCrott@users.noreply.github.com>
 Nigel Tao <nigeltao@golang.org>
+Peter Kasting <pkasting@chromium.org>