doc/go1.7.html: earlier Go versions don't work on macOS Sierra

Updates #16272.

Change-Id: If5444b8de8678eeb9be10b62a929e2e101d1dd91
Reviewed-on: https://go-review.googlesource.com/24900
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.7.html

@@ -74,6 +74,12 @@ <h2 id="language">Changes to the language</h2>
 
 <h2 id="ports">Ports</h2>
 
+<p>
+Go 1.7 adds support for macOS 10.12 Sierra.
+Binaries built with previous versions of Go will not work correctly on
+Sierra.
+</p>
+
 <p>
 Go 1.7 adds an experimental port to <a href="https://en.wikipedia.org/wiki/Linux_on_z_Systems">Linux on z Systems</a> (<code>linux/s390x</code>)
 and the beginning of a port to Plan 9 on ARM (<code>plan9/arm</code>).