[release-branch.go1.14] doc/go1.14: add link to module migration guide

Adding a link to this guide will provide more value to instructing Go
users to migrate to modules.

Updates #36878

Change-Id: Ie6ab45efcd35cc5e5ba5adc16ba0ca4cca4292bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/220906
Run-TryBot: Alexander Rakoczy <alex@golang.org>
Reviewed-by: thepudds <thepudds1460@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 8e2dad5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/220981
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: toothrot

doc/go1.14.html

@@ -25,9 +25,9 @@ <h2 id="introduction">Introduction to Go 1.14</h2>
 
 <p>
   Module support in the <code>go</code> command is now ready for production use,
-  and we encourage all users to migrate to Go modules for dependency management.
-  If you are unable to migrate due to a problem in the Go toolchain,
-  please ensure that the problem has an
+  and we encourage all users to <a href="https://blog.golang.org/migrating-to-go-modules">migrate to Go
+  modules for dependency management</a>. If you are unable to migrate due to a problem in the Go
+  toolchain, please ensure that the problem has an
   <a href="https://golang.org/issue?q=is%3Aissue+is%3Aopen+label%3Amodules">open issue</a>
   filed. (If the issue is not on the <code>Go1.15</code> milestone, please let us
   know why it prevents you from migrating so that we can prioritize it