Add paypal link

doc/gmsh.html

@@ -280,6 +280,18 @@ href="http://www.cs.berkeley.edu/~jrs/triangle.shar.gz">here</a>.
 Gmsh's high quality vector PostScript output is produced by <a
 href="/gl2ps/">GL2PS</a>.
 
+<p>
+<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
+<input type="hidden" name="cmd" value="_xclick">
+<input type="hidden" name="business" value="geuzaine@acm.caltech.edu">
+<input type="hidden" name="item_name" value="Gmsh donation">
+<input type="hidden" name="no_note" value="1">
+<input type="hidden" name="currency_code" value="USD">
+<input type="hidden" name="tax" value="0">
+<input type="image" src="/images/donation.gif" border="0" name="submit" 
+       alt="Make a donation with PayPal">
+</form>
+
 <p>
 Back to <a href="/">geuz.org</a>