doc: preannounce dropping macOS 10.10 support

Updates #23011

Change-Id: I0eccea5d08a8758585f183540787b78fb80aa36a
Reviewed-on: https://go-review.googlesource.com/c/151360
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -47,6 +47,11 @@ <h3 id="freebsd">FreeBSD</h3>
   12.0+.
 </p>
 
+<p id="darwin">
+Go 1.12 is the last release that will run on macOS 10.10 Yosemite.
+Go 1.13 will require macOS 10.11 El Capitan or later.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="gocache">Build cache requirement</h3>