doc/go1.10: preannounce removal of OS X 10.8 support in Go 1.11

For #23011.

Change-Id: Ibd934f32e41cb8dddbd3d68a324536cf29a97916
Reviewed-on: https://go-review.googlesource.com/82275
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.10.html

@@ -64,6 +64,11 @@ <h2 id="ports">Ports</h2>
 and improvements to the code generated by the compilers.
 </p>
 
+<p id="darwin">
+Go 1.10 is the last release that will run on OS X 10.8 Mountain Lion.
+Go 1.11 will require OS X 10.9 Mavericks or later.
+</p>
+
 <p id="freebsd">
 As <a href="go1.9#freebsd">announced in the Go 1.9 release notes</a>,
 Go 1.10 now requires FreeBSD 10.3 or later;