CONTRIBUTORS: add David Covert (Google CLA)

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/71670043

CONTRIBUTORS

@@ -162,6 +162,7 @@ Dave Day <djd@golang.org>
 Dave Grijalva <dgrijalva@ngmoco.com>
 David Anderson <danderson@google.com>
 David Bürgin <676c7473@gmail.com>
+David Covert <davidhcovert@gmail.com>
 David Crawshaw <david.crawshaw@zentus.com> <crawshaw@google.com>
 David du Colombier <0intro@gmail.com>
 David Forsythe <dforsythe@gmail.com>