maintner: document that GitHubIssue.ClosedBy field is not implemented

The ClosedBy field is currently always nil due to the cause described
in the linked issue. Document it with a TODO comment so people don't
need to spend time on figuring that out for themselves.

Updates golang/go#28745

Change-Id: Icaa7b8fd5614dffbfd13a9783b9a71cb87e2af40
Reviewed-on: https://go-review.googlesource.com/c/149238
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: dmitshur

maintner/github.go

@@ -240,7 +240,7 @@ type GitHubIssue struct {
 	Created     time.Time
 	Updated     time.Time
 	ClosedAt    time.Time
-	ClosedBy    *GitHubUser
+	ClosedBy    *GitHubUser // TODO(dmitshur): Implement (see golang.org/issue/28745).
 	Title       string
 	Body        string
 	Milestone   *GitHubMilestone       // nil for unknown, noMilestone for none