Mention USB size in system requirements (Will-fix: #16810)

wiki/src/doc/about/requirements.mdwn

@@ -20,6 +20,10 @@ issues|support/known_issues]].</p>
 
 Hardware requirements:
 
+- A USB stick of 8 GB minimum or a DVD recordable.
+
+  All the data on this USB stick or DVD will be lost when installing Tails.
+
 - The possibility to start from a USB stick or a DVD reader.
 
 - A 64-bit <span class="definition">[[!wikipedia x86-64]]</span>