phpmailer: add a README file listing our customisations

Signed-off-by: Francois Marier <francois@catalyst.net.nz>

htdocs/lib/phpmailer/README.Mahara

+phpmailer in Mahara
+======================
+
+Website: http://sourceforge.net/projects/phpmailer/
+Version: 5.0.2
+
+This library is for the email_user() function in lib/user.php
+
+Changes:
+
+ * None