internal/history: document upcoming Go 1.15.5 and Go 1.14.12 security releases

They have been pre-announced¹.

¹ https://groups.google.com/d/msg/golang-announce/kMa3eup0qhU/O5RSMHO_CAAJ

Change-Id: I90e5844d9f485a45790ad0f39b42091df95ac061
Reviewed-on: https://go-review.googlesource.com/c/website/+/268617
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
diff --git a/internal/history/release.go b/internal/history/release.go
index 08ce0a6..c0d521a 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, 5}: {
+		Date:     Date{2020, 11, 12},
+		Security: true,
+		Future:   true,
+	},
+	{1, 14, 12}: {
+		Date:     Date{2020, 11, 12},
+		Security: true,
+		Future:   true,
+	},
+
 	{1, 15, 4}: {
 		Date: Date{2020, 11, 5},