CONTRIBUTORS: add Andreas Krebbel (IBM CLA)
CONTRIBUTORS: add Andreas Krebbel (IBM CLA)

Change-Id: I850e16a4005a6a0142463a29ed48ea41fcf020f8
Reviewed-on: https://go-review.googlesource.com/26993
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend