GoUsers: add Reface to Ukraine section

This Ukrainian startup has a lot of backend services written in Go.

Change-Id: I190a784cae80e3b94742cceb3086396e66db156e
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/644595
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>