internal/history: document Go 1.15.5 and Go 1.14.12

Change-Id: Id06cb288888847ff4bb9251681b6d7a6d1f9381b
Reviewed-on: https://go-review.googlesource.com/c/website/+/269637
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

internal/history/release.go

diff --git a/internal/history/release.go b/internal/history/release.go
index c0d521a..2d9db8a 100644
--- a/internal/history/release.go
+++ b/internal/history/release.go
@@ -34,12 +34,12 @@
 	{1, 15, 5}: {
 		Date:     Date{2020, 11, 12},
 		Security: true,
-		Future:   true,
+		Packages: []string{"cmd/go", "math/big"},
 	},
 	{1, 14, 12}: {
 		Date:     Date{2020, 11, 12},
 		Security: true,
-		Future:   true,
+		Packages: []string{"cmd/go", "math/big"},
 	},
 
 	{1, 15, 4}: {