cmd/go: release note for -mod=readonly by default

For #40728
Fixes #42466

Change-Id: If2b21b37a590c243828c4fd278ab10b2705450f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/268859
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.16.html

@@ -82,6 +82,15 @@ <h3 id="go-command">Go command</h3>
 
 <h4 id="modules">Modules</h4>
 
+<p><!-- golang.org/issue/40728 -->
+  Build commands like <code>go</code> <code>build</code> and <code>go</code>
+  <code>test</code> no longer modify <code>go.mod</code> and <code>go.sum</code>
+  by default. Instead, they report an error if a module requirement or checksum
+  needs to be added or updated (as if the <code>-mod=readonly</code> flag were
+  used). Module requirements and sums may be adjusted with <code>go</code>
+  <code>mod</code> <code>tidy</code> or <code>go</code> <code>get</code>.
+</p>
+
 <p><!-- golang.org/issue/40276 -->
   <code>go</code> <code>install</code> now accepts arguments with
   version suffixes (for example, <code>go</code> <code>install</code>