doc: document Go 1.12.7

Change-Id: Id5d2f4cc6bc310bed2516ce0f50c395802475f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/185258Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/devel/release.html

@@ -77,6 +77,13 @@ the go command, and the <code>crypto/x509</code>, <code>net/http</code>, and
 1.12.6 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.7 (released 2019/07/08) includes fixes to cgo, the compiler,
+and the linker.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.7">Go
+1.12.7 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>