doc: document Go 1.12.5

Change-Id: I9986a323db2a8f5fa74b071cfd04e8c786da0cb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/175438Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/devel/release.html

@@ -62,6 +62,13 @@ when linking programs that used cgo.
 Only Linux users who hit this issue need to update.
 </p>
 
+<p>
+go1.12.5 (released 2019/05/06) includes fixes to the compiler, the linker,
+the go command, the runtime, and the <code>os</code> package. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.5">Go
+1.12.5 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>