internal/history: document upcoming Go 1.15.1 and Go 1.14.8 security releases

They have been pre-announced¹.

¹ https://groups.google.com/d/msg/golang-announce/JvvJpgiIfFI/99z802JLAwAJ

Change-Id: I03712ae6af76770970679ea5e53c7f5238cce9f2
Reviewed-on: https://go-review.googlesource.com/c/website/+/251119
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
diff --git a/internal/history/release.go b/internal/history/release.go
index c7ead9f..33000f1 100644
--- a/internal/history/release.go
+++ b/internal/history/release.go
@@ -31,6 +31,17 @@
 // 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, 15, 1}: {
+		Date:     Date{2020, 9, 1},
+		Security: true,
+		Future:   true,
+	},
+	{1, 14, 8}: {
+		Date:     Date{2020, 9, 1},
+		Security: true,
+		Future:   true,
+	},
+
 	{1, 15, 0}: {
 		Date: Date{2020, 8, 11},
 	},