Adding info about installing the adodb c lib (Bug #1427046)

To test - read the new instructions in the README file and try
installing it on your machine and then test to make sure Mahara is not
broken.

Advanced test - install xhprof and make sure the use of this lib does
in fact make things faster.

Change-Id: Ia4aff333df7353d0aa40c771264e4c1be7d94f51
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>

README

@@ -55,6 +55,7 @@ Mahara 1.11 requires:
  ** openssl and xmlrpc (optional; for networking support)
  ** zip (optional)
  ** zlib (optional)
+ ** adodb (optional; improves performance)
  ** ... and ensure that the magic_quotes and register_globals settings are off.
 
  * OS: Mahara is only officially supported on Debian (5.0/"Lenny" or later) and