Add instructions on importing signing key into GPGTools

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

@@ -364,6 +364,12 @@ signing key through the OpenPGP Web of Trust</a>.</p>
 
 <h3>In macOS using <span class="application">GPGTools</span></h3>
 
+<p>Download the [[Tails signing key|tails-signing.key]] and import it into
+<span class="application">GPGTools</span>.</p>
+
+<p>See the [[<span class="application">GPGTools</span> documentation on
+importing keys|https://gpgtools.tenderapp.com/kb/gpg-keychain-faq/how-to-find-public-keys-of-your-friends-and-import-them#import-key-file]].</p>
+
 <ol>
   <li>
    Open <span class="application">Finder</span> and navigate to the