pkg/errors: add comment to exported function (#676)

pkg/errors/gocmd.go

@@ -4,6 +4,8 @@ import (
 	"strings"
 )
 
+// IsRepoNotFoundErr returns true if the Go command line
+// hints at a repository not found.
 func IsRepoNotFoundErr(err error) bool {
 	return strings.Contains(err.Error(), "remote: Repository not found")
 }