Add tip about using discount codes

Thanks to Ralphis for his comment on Doomworld that prompted me to add
this.

Author: fragglet

manual.html

@@ -109,6 +109,12 @@ <h2 id="_printed_manuals">Printed manuals</h2>
 </p>
 </li>
 </ul></div>
+<div class="paragraph"><p><strong>Tip</strong>: You can sometimes save 10-20% on your purchase if you find a Lulu.com
+discount code. Check for recent posts on
+<a href="https://www.facebook.com/Luludotcom">their Facebook page</a> or
+<a href="https://www.retailmenot.com/view/lulu.com">RetailMeNot</a> and you may get
+lucky. Please note that the codes are case sensitive: if the code is in ALLCAPS
+then it&#8217;s important you type it that way.</p></div>
 </div>
 </div>
 <p></p>

manual.txt

@@ -22,3 +22,10 @@ below to purchase your copy.
   * link:https://www.lulu.com/shop/simon-howard/freedoom-manual-english/paperback/product-zmykn7d.html[Freedoom Manual (English)]
   * link:https://www.lulu.com/shop/simon-howard/freedoom-manual-espa%C3%B1ol/paperback/product-9596pqz.html[Freedoom Manual (Español)]
   * link:https://www.lulu.com/shop/simon-howard/freedoom-manual-fran%C3%A7ais/paperback/product-rm6zygy.html[Freedoom Manual (Français)]
+
+**Tip**: You can sometimes save 10-20% on your purchase if you find a Lulu.com
+discount code. Check for recent posts on
+link:https://www.facebook.com/Luludotcom[their Facebook page] or
+link:https://www.retailmenot.com/view/lulu.com[RetailMeNot] and you may get
+lucky. Please note that the codes are case sensitive: if the code is in ALLCAPS
+then it's important you type it that way.