Point to documentation about creating OpenPGP keys

This scenario will be advertised as "using the command line and GnuPG"
which should filter out people quite a bit. So people actually opening
this page could be interested in learning how to do this.

wiki/src/install/expert/usb.mdwn

@@ -10,8 +10,18 @@ Verify the Tails signing key
 
 In this step, you will download and verify the <em>Tails signing
 key</em> which is the OpenPGP key that is used to cryptographically sign
-the Tails ISO image. To follow these instructions you need to have an
-OpenPGP key yourself.
+the Tails ISO image.
+
+<div class="note">
+
+<p>To follow these instructions you need to have your own OpenPGP
+key.</p>
+
+<p>To learn how to create yourself an OpenPGP key, see
+<a href="https://help.riseup.net/en/security/message-security/openpgp/gpg-keys">Managing
+OpenPGP Keys</a> by Riseup.</p>
+
+</div>
 
 <div class="tip">