adodb: add a README file listing our customisations

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

htdocs/lib/adodb/README.mahara

+adodb
+-----
+
+Source: http://adodb.sourceforge.net/
+Version: 4.92
+
+Modifications:
+
+- drivers/adodb-mysql.inc.php (d2b9a10cfa9a64e1cc1a0691adce71bf4203e1d3)
+- drivers/adodb-postgres7.inc.php (4652e5617d9616e3f61e78df5a4c863e16ac5634)
+- adodb/drivers/adodb-postgres64.inc.php (4bdafccba511540e6db92277aa104a23f57209ac)