Update README to reflect mbstring dependency (non-default extension)

When installing Tracy on a Windows server, the debugger bar did not show, despite it working fine on my Mac. I tracked the error to `mbstring` not being enabled. The [PHP documentation](http://www.php.net/manual/en/mbstring.installation.php) describes the extension as non-default, so I updated the README to help others who hit the same hiccup.