[release-branch.go1.12] doc: add minor revisions header for 1.12

Change-Id: I0d4a55af0bee754ab1ee817780027e9f72475afb
Reviewed-on: https://go-review.googlesource.com/c/go/+/167712Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/devel/release.html

@@ -30,6 +30,8 @@ Go 1.12 is a major release of Go.
 Read the <a href="/doc/go1.12">Go 1.12 Release Notes</a> for more information.
 </p>
 
+<h3 id="go1.12.minor">Minor revisions</h3>
+
 <p>
 go1.12.1 (released 2019/03/14) includes fixes to cgo, the compiler, the go
 command, and the <code>fmt</code>, <code>net/smtp</code>, <code>os</code>,