internal/history: document upcoming Go 1.16.1 and 1.15.9 security releases

They have been pre-announced¹.

¹ https://groups.google.com/g/golang-announce/c/UERZo89zw8o

Change-Id: Ief02b0a7ced6b69a2aef7b1b0cf288b8df7116f8
Reviewed-on: https://go-review.googlesource.com/c/website/+/299270
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
diff --git a/internal/history/release.go b/internal/history/release.go
index 7cd3aab..a9cc3f4 100644
--- a/internal/history/release.go
+++ b/internal/history/release.go
@@ -31,6 +31,19 @@
 // It contains entries for releases of Go 1.9 and newer.
 // Older releases are listed in doc/devel/release.html.
 var Releases = map[GoVer]Release{
+	{1, 16, 1}: {
+		Date:     Date{2021, 3, 10},
+		Security: true,
+
+		Future: true,
+	},
+	{1, 15, 9}: {
+		Date:     Date{2021, 3, 10},
+		Security: true,
+
+		Future: true,
+	},
+
 	{1, 16, 0}: {
 		Date: Date{2021, 2, 16},
 	},