doc: add release notes describing the default mirror and sumdb

Change-Id: I4923f0726ae0261a7c7b0f85e7433ae0f605c123
Reviewed-on: https://go-review.googlesource.com/c/go/+/181738Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go1.13.html

@@ -24,6 +24,15 @@ Do not send CLs removing the interior tags from such phrases.
   </strong>
 </p>
 
+<p>
+  As of Go 1.13, the go command downloads and authenticates modules using
+  the Go module mirror and Go checksum database run by Google. See
+  <a href="https://proxy.golang.org/privacy">https://proxy.golang.org/privacy</a>
+  for privacy information about these services and the
+  <a href="/cmd/go/#hdr-Module_downloading_and_verification">go command documentation</a>
+  for configuration details.
+</p>
+
 <p>
 TODO
 </p>