Add explanation why we need another USB stick to upgrade

wiki/src/install/inc/overview.html

@@ -49,7 +49,7 @@
         </div>
       </div>
     </div>
-    <div class="col-sm-6 col-md-3 debian expert clone mac-dvd upgrade-tails">
+    <div class="col-sm-6 col-md-3 debian expert clone mac-dvd">
       <div class="thumbnail">
         [[!img install/inc/icons/1-usb.png link="no"]]
         <div class="caption">
@@ -60,6 +60,23 @@
         </div>
       </div>
     </div>
+    <div class="col-sm-6 col-md-3 upgrade-tails">
+      <div class="thumbnail">
+        [[!img install/inc/icons/1-usb.png link="no"]]
+        <div class="caption">
+          <h4>1 USB stick<br />
+            <small>4 GB minimum</small> <br />
+            <small>All data will be lost!</small>
+            [[!toggle id="why_extra" text="Why another one?"]]
+
+            [[!toggleable id="why_extra" text="""
+            [[!toggle id="why_extra" text="Close"]]
+            [[!inline pages="install/inc/router/why_extra.inline" raw="yes"]]
+            """]]
+          </h4>
+        </div>
+      </div>
+    </div>
     <div class="col-sm-6 col-md-3 mac-dvd">
       <div class="thumbnail">
         [[!img install/inc/icons/1-dvd.png link="no"]]

wiki/src/install/inc/router/why_extra.inline.html

+<p>It is currently impossible to manually upgrade a Tails USB stick
+while running from itself. This scenario requires creating an
+intermediary Tails on another USB stick, from which to upgrade your
+Tails.</p>

wiki/src/upgrade.html

@@ -39,11 +39,16 @@
 
     <ul>
       <li>Your USB stick</li>
-      <li>Another empty USB stick <small>(at least 4 GB)</small></li>
+      <li>Another empty USB stick <small>(at least 4 GB)</small>[[!toggle id="why_extra" text="Why another one?"]]</li>
       <li>30&ndash;60 minutes to download Tails <small>[[!inline pages="inc/stable_i386_iso_size" raw="yes"]]</small></li>
       <li>20 minutes to upgrade</li>
     </ul>
 
+    [[!toggleable id="why_extra" text="""
+    [[!toggle id="why_extra" text="Close"]]
+    [[!inline pages="install/inc/router/why_extra.inline" raw="yes"]]
+    """]]
+
     [[
       <div class="panel panel-primary">
         <div class="panel-heading">