Comment 3 for bug 1519516

Okay, the README.md file points users to the wiki for upgrade advice. And the wiki *already* tells you that extracting the new code over the old code is not recommended because of these sorts of issues.

I've updated it make this even clearer, and I've added a section to the FAQ at the end, with the specific PHP error caused when you upgrade to 15.10 in this way: https://wiki.mahara.org/wiki/System_Administrator's_Guide/Upgrading_Mahara#Q:_PHP_Fatal_error:_Cannot_redeclare_get_message_thread_mr.28.29