[ticket/11996] Add explanation how to correctly remove and update an extension

See http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=44001 for more
information about the decision.

PHPBB3-11996
[ticket/11996] Use a table for better styling

PHPBB3-11996
[ticket/11996] Remove note until we really do disable the board

PHPBB3-11996
Merge pull request #1942 from nickvergessen/ticket/11996

[ticket/11996] Add explanation how to correctly remove and update an extension