Add instructions on importing signing key into Gpg4win

wiki/src/install/inc/steps/download.inline.html

@@ -336,6 +336,12 @@ BitTorrent.</p>
 
 <h3>In Windows with <span class="application">Gpg4win</span></h3>
 
+<p>Download the [[Tails signing key|tails-signing.key]] and import it into
+<span class="application">Gpg4win</span>.</p>
+
+<p>See the [[<span class="application">Gpg4win</span> documentation on
+importing keys|https://www.gpg4win.org/doc/en/gpg4win-compendium_15.html]].</p>
+
 <p>See the [[<span class="application">Gpg4win</span> documentation on
 verifying signatures|http://www.gpg4win.org/doc/en/gpg4win-compendium_24.html#id4]].</p>