doc: announce the end of support for binary-only packages

Updates #28152

Change-Id: If859221afc683b392f649e79d7ff0a06125cbe10
Reviewed-on: https://go-review.googlesource.com/c/152918Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -98,6 +98,12 @@ Go 1.13 will require macOS 10.11 El Capitan or later.
   has no effect in Go 1.12.
 </p>
 
+<h3 id="binary-only">Binary-only packages</h3>
+
+<p>
+  Go 1.12 is the last release that will support binary-only packages.
+</p>
+
 <h3 id="modules">Modules</h3>
 
 <p><!-- CL 148517 -->