cmd/golangorg,internal/history: document Go 1.16.4 and Go 1.15.12

1.16.4 and 1.15.12 are our first releases containing public security
fixes. As such, the existing template doesn't work well for them, so use
a CustomSummary.

Change-Id: Ib70990425b1a2963030b1034295a20b4c105c630
Reviewed-on: https://go-review.googlesource.com/c/website/+/317609
Trust: Heschi Kreinick <heschi@google.com>
Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
diff --git a/internal/history/release.go b/internal/history/release.go
index 63a5368..c97737f 100644
--- a/internal/history/release.go
+++ b/internal/history/release.go
@@ -31,6 +31,26 @@
 // 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, 4}: {
+		Date: Date{2021, 5, 6},
+
+		CustomSummary: `includes a security fix to the
+<code>net/http</code> package, as well as bug fixes to the runtime,
+the compiler, and the <code>archive/zip</code>, <code>time</code>,
+and <code>syscall</code> packages. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.16.4+label%3ACherryPickApproved">Go
+1.16.4 milestone</a> on our issue tracker for details.`,
+	},
+	{1, 15, 12}: {
+		Date: Date{2021, 5, 6},
+
+		CustomSummary: `includes a security fix to the
+<code>net/http</code> package, as well as bug fixes to the runtime and
+the <code>time</code> package. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.15.12+label%3ACherryPickApproved">Go
+1.15.12 milestone</a> on our issue tracker for details.`,
+	},
+
 	{1, 16, 3}: {
 		Date: Date{2021, 4, 1},